The reveal of Palpatine certainly must have caught plenty of fans by surprise, but when you think about it, there weren’t many options available to The Rise of Skywalker for villains. After the death of Snoke, there’s really no one left. Yes, there’s Kylo Ren, but his story is so tied up in the will he/won’t he be a good guy, that he doesn’t really feel like a final boss. You can’t introduce a new guy because what good would that do in the last film? So, you bring back the guy everyone knows and it becomes a lot stronger.
Article source: https://www.cinemablend.com/news/2470288/ian-mcdiarmid-reacts-to-palpatines-return-the-dark-side-is-stronger-than-ever