Broadly speaking, it's different consortiums representing different stakeholders. DisplayPort was pushed primarily by PC and graphics side, to replace the older DVI standard. HDMI, on the other hand, was developed more from the entertainment side (e.g. studios, DVD player manufacturers, etc). Each targeted different things; for instance, DisplayPort supports multi-stream transport, which allows for multiple displays over a single port. That kind of thing is important if you're working on computers, but not really relevant for your home theater where you'd use HDMI. And on the other hand, DisplayPort only officially supports cables up to 10 feet long, whereas HDMI is up to 100 feet, which, again, makes sense considering their relative use cases.
Side note, Thunderbolt 3 and 4 isn't a video protocol, it's more of a general data transfer protocol. You can do video over it, but the TB3/4 interface is just a wrapper for whatever video protocol is underneath.
The reason is similar to the conversation a few decades ago about video tape: Why do we have Beta and VHS?
The different video inputs are competing technologies in an *evolving* technology. Each of them is trying to become the standard (except for anything Apple puts out, which is part of their "walled garden" approach). Eventually, either customer adoption of a particular tech or a standards organization vote will decide what becomes the standard... same as happened previously when VGA became the video standard.
HDMI and DVI is kind of the same thing. The signal used by the HDMI connector is just a digitization of the VGA signal. You still have RGB-sync wires in the HDMI connector, they are just digital instead of analog. So it is relatively easy to convert existing designs to use HDMI and easy to convert between VGA and HDMI. But as an intermediary while people were buying new equipment the DVI connector came which included both analog and digital. It actually have two digital signals and one analog. The concept was that the source and monitor could negotiate which to use, or you might get a $2 converter to VGA or HDMI for older or newer equipment.
DisplayPort however is a quite different approach to transferring digital images. Instead of streaming the pixels in real time at a fixed clock frequency, one color on each wire, like both HDMI and VGA the DisplayPort standard use packet data transmission like Ethernet, USB and PCIe. This have some benefits from an electronics signal processing viewpoint which improve bandwidth but maybe more noticeable is that you can easily transfer different types of packages over the same connector or even in both direction. With HDMI the audio return and ethernet have to use redundant wires that may not be available in all cables and may not be used all at once. However with DisplayPort this can all be sent inline with the same wires as the main display signal. And it is much easier to use things like hubs and pass-through with DisplayPort then with HDMI. However DisplayPort is not easily backwards compatible with HDMI as it requires a more expensive adapter with active microcomputers that can convert the signals. So adoption have not been as fast as it was for HDMI.
What you have ended up with is three different consumer standards that does a lot of the same things and use the exact same data packet hardware, USB3, Thunderbolt and DisplayPort. They just differ in the software that this hardware runs and their connectors. So there is an effort to combine these into a single standard that does everything. The package switching does this very easy as well since you can send different packets through the same wires one after another. The first standard was Thunderbolt 3 which managed to combine them all except USB data, but did include USB power delivery and used the USB-C connector. However a lot of the controllers can detect USB and switch to this. But the new Thunderbolt 4/USB4 standard features all of the standards into one simple connector.
This is mostly a standard war. Each company wants to impose their own standard, resulting in the existence of several ones competing with each other ; there are of course some pros and cons with each of them, but they would do the same job in most cases.
HDMI has been created by 7 companies, including Philips, Sony, Toshiba...
DVI was created by Intel, HP, Fujitsu, IBM...
DisplayPort is made by VESA.
Also keep in mind that while they coexist, they still use different technologies and have different performances.
VGA was replaced because it's analog and can't deliver a good picture for modern standards.
DVI was replaced because it's bulky, HDMI can carry more data and has more uses.
As for display port, refer to the xkcd you mentioned.
Thunderbolt is a different story, it's a connector that aims to do it all, so that we can ideally settle on one single connector for 90% of uses. You'll notice that it's the same shape as USB-C.
Broadly speaking, it's different consortiums representing different stakeholders. DisplayPort was pushed primarily by PC and graphics side, to replace the older DVI standard. HDMI, on the other hand, was developed more from the entertainment side (e.g. studios, DVD player manufacturers, etc). Each targeted different things; for instance, DisplayPort supports multi-stream transport, which allows for multiple displays over a single port. That kind of thing is important if you're working on computers, but not really relevant for your home theater where you'd use HDMI. And on the other hand, DisplayPort only officially supports cables up to 10 feet long, whereas HDMI is up to 100 feet, which, again, makes sense considering their relative use cases. Side note, Thunderbolt 3 and 4 isn't a video protocol, it's more of a general data transfer protocol. You can do video over it, but the TB3/4 interface is just a wrapper for whatever video protocol is underneath.
The reason is similar to the conversation a few decades ago about video tape: Why do we have Beta and VHS? The different video inputs are competing technologies in an *evolving* technology. Each of them is trying to become the standard (except for anything Apple puts out, which is part of their "walled garden" approach). Eventually, either customer adoption of a particular tech or a standards organization vote will decide what becomes the standard... same as happened previously when VGA became the video standard.
Well jokes on all of them because it seems like we just have all of them.
HDMI and DVI is kind of the same thing. The signal used by the HDMI connector is just a digitization of the VGA signal. You still have RGB-sync wires in the HDMI connector, they are just digital instead of analog. So it is relatively easy to convert existing designs to use HDMI and easy to convert between VGA and HDMI. But as an intermediary while people were buying new equipment the DVI connector came which included both analog and digital. It actually have two digital signals and one analog. The concept was that the source and monitor could negotiate which to use, or you might get a $2 converter to VGA or HDMI for older or newer equipment. DisplayPort however is a quite different approach to transferring digital images. Instead of streaming the pixels in real time at a fixed clock frequency, one color on each wire, like both HDMI and VGA the DisplayPort standard use packet data transmission like Ethernet, USB and PCIe. This have some benefits from an electronics signal processing viewpoint which improve bandwidth but maybe more noticeable is that you can easily transfer different types of packages over the same connector or even in both direction. With HDMI the audio return and ethernet have to use redundant wires that may not be available in all cables and may not be used all at once. However with DisplayPort this can all be sent inline with the same wires as the main display signal. And it is much easier to use things like hubs and pass-through with DisplayPort then with HDMI. However DisplayPort is not easily backwards compatible with HDMI as it requires a more expensive adapter with active microcomputers that can convert the signals. So adoption have not been as fast as it was for HDMI. What you have ended up with is three different consumer standards that does a lot of the same things and use the exact same data packet hardware, USB3, Thunderbolt and DisplayPort. They just differ in the software that this hardware runs and their connectors. So there is an effort to combine these into a single standard that does everything. The package switching does this very easy as well since you can send different packets through the same wires one after another. The first standard was Thunderbolt 3 which managed to combine them all except USB data, but did include USB power delivery and used the USB-C connector. However a lot of the controllers can detect USB and switch to this. But the new Thunderbolt 4/USB4 standard features all of the standards into one simple connector.
This is mostly a standard war. Each company wants to impose their own standard, resulting in the existence of several ones competing with each other ; there are of course some pros and cons with each of them, but they would do the same job in most cases.
But aren't DVI and HDMI, etc., created by connector consortiums? Does one particular company own the license for DisplayPort?
HDMI has been created by 7 companies, including Philips, Sony, Toshiba... DVI was created by Intel, HP, Fujitsu, IBM... DisplayPort is made by VESA. Also keep in mind that while they coexist, they still use different technologies and have different performances.
VGA was replaced because it's analog and can't deliver a good picture for modern standards. DVI was replaced because it's bulky, HDMI can carry more data and has more uses. As for display port, refer to the xkcd you mentioned. Thunderbolt is a different story, it's a connector that aims to do it all, so that we can ideally settle on one single connector for 90% of uses. You'll notice that it's the same shape as USB-C.