Why did Monero Split?

Think it's just to try and stop ASIC's mining it TBH.
It's a good move if they want more people to mine it using their computer processors. It will also help keep things like Coinhive afloat, which Nulled uses for their mining feature, though it's hardly profitable.
I think that Coinhive might move to the new branch once it's complete (if it hasn't already)