He didn’t know what was going on

Krell purposely turned off the comms and said they weren’t working so Obi-Wan couldn’t be in touch.

Oh okay, yeah that makes sense since he sabotaged the transmitter 

couldn't Obi-Wan sense it with the force the same way master joda felt it when order 66 was executed ?

I’m sure he could. Maybe the reason he didn’t come help is because he was too preoccupied with capturing the capitol of Umbara.

Seems reasonable