But there are people who didn't play WoW back in the day who still love classic, so it can't just be nostalgia. Vanilla WoW really did have a different design ethos than the later expansions did, and some people prefer that experience.

Another example is Old School Runescape, who reverted back to an earlier save and has now diverged as an entirely separate game running with older systems as they lost a ton of players with their "Evolution of Combat" update. While nostalgia is definitely a powerful tool, I agree with the previous commenter that the original WoW was a very different game than the modern version and it seems like that is one of the core aspects of what people desired.