Why? I don't know.
For me, part of it is definitely nostalgia since I've been watching for thirty-five years. Part of it is cool adventure with fist fights and feats of daring-do. Part of it is cool spaceships hurtling massive destructive forces at each other. Part of it is Captain Kirk, every kid and men who are kids at heart want to punch the bad guy, save the universe and get the girl. Part of it is arguing with other fans who see the universe differently, they should make debating the Prime Directive an Olympic sport.
Lots of different reasons why I like Star Trek.