Translation: Well, ya see, Rian really shit the bed by killing off the big bad in the second part of a trilogy. We had no other big villains we could build up in one movie.
Kylo already got his ass handed to him by Rey in Force Awakens and was a whiny crybaby bitch in Last Jedi, so he wouldn't work. Hux was made to look like a buffoon throughout all of Last Jedi and would also pose no threat to Rey.
So that's when I said 'fuck it! Let's just use the big bad from the original trilogy!' This is genius see, we never actually saw a corpse and this would make nerds wet themselves with excitement!
There was talk of gender swapping the Emperor, but Kathleen correctly called out my blatant misogynistic views by pointing out that women can never be bad and here we are!!