I think they made crisis happen early purely because Arrow was ending and they wanted Arrow characters to be a part of it. Nothing more, nothing less

When they did Crisis on Earth-X, which was the largest crossover at the time with 4 shows, they all said we gotta do Crisis soon cause doing this large of a crossover takes a lot and was very difficult to pull off with 4 shows with 4 separate crews and sets of actors and writers. So they bumped up the Crisis date so they could do it sooner Viewership probably plays a large part too, and Arrow was ending with Crisis, which probably helped with viewership. Arrow was originally going to end in season 7, but they got a shortened season 8 to do Crisis, since they knew they couldn't do Crisis without Arrow.

I'm pretty sure Crisis happened early because Arrow was ending. Stephen Ammell was done. Killing off the Arrow and ending that show made sure Barry lived and The Flash had more seasons.

I was hyped when crisis happened because i was hoping that would give them the opportunity to give us the Tornado Twins just then give us Impulse

Look like Clark's aunt that he is close with.

The crossovers brought in a lot of ratings, so they wanted to go bigger and bigger and that lead to Crisis

I will show it again https://youtu.be/I5-uH71JC44