Why couldn't Snoke just be some evil Sith Lord that rose to power after the fall of the empire and got killed due to his own hubris like every other Sith Lord ever. Why do we need to connect him to Palpatine too.
Bor Gullet won. This whole universe just revolves around fucking Sheev.
Making Snoke a Palps pawn is an easy way to explain the character away while moving foward with the plot.
The details will all be filled out in a upcoming comic I'm sure.