Snoke’s rise to power isn’t important in the ST though. Most of all the information you need for the story that’s being told is there.
The simple fact is, TFA is not a direct sequel to ROTJ. It’s more so it’s own story that takes place after ROTJ.
It is a sequel to ROTJ. The fact that it takes place so many years later is all the more reason to set the scene. It’s a cop out to expect the audience to accept anything just because a lot of time passed. It’s not important to the story how 3PO got a red arm and cosmetic changes are expected. It is important how the Empire/FO recovered under the rule of a Sith(?) after it appeared thoroughly beaten.