I hate to get dragged into discussion about this mediocre movie but you all know there's a difference. A New Hope came with a fully established universe with an evil empire already in place, one that the audience at the time could even assume existed long before its current rulers (the Emperor and Vader) came into play.
So it makes sense that we don't know where they came from, just as the fact we don't need to know where Hans Gruber came from in Die Hard to make him the man he is.
TFA and this however are direct follow-ups to the OG trilogy so to have an all-powerful character present without explaining how he rose to power following the events of RotJ just feels off. I mean, they could've given him as much as a single line about how he had to crawl out of the ashes of the emperor's failure to explain that this guy at least existed during the OG trilogy's events.