You’re wrong. So much backstory went into Palpatine BEFORE the original trilogy was written. As I said before, you can find much of that backstory in the preface of the Star Wars novelization. It doesn’t have to be told onscreen. Writers write backstories as a tool and guideline to help them write about the characters current motivations. The fact is none of this sort was written about Snoke - on or offscreen. But the detailed history of Palpatine was written before the movies.
About Zahn and Lucas, they had a plan on where the movies were going. Lucas made sure that Zahn didn’t step on that plan, making sure that characters, even, that were planned for Lucas’ sequels had the same names and backgrounds as Zahn’s.
Then, Disney just decided to ignore all those years of work for no logical rhyme or reason.
Even Rian Johnson decided to honor that canon to some extent by giving Kilo Ren his original name back — Ben Solo.