It's all about capitalism.

Back then games where made for a different audience. That, combined with the demographics of the players + some technological limitations mean that each wow expansion is a different game.

I can speak more clearly about Diablo. The distinction between each game is absolutely massive, being of the same franchise means nothing . The people who enjoy one them and hate the others do it for very,very different reasons.

In the end, it's about you. Each expansion will appeal to a different type of player. Vanilla is loved because it is (comparatively) brutal. Only the good and dedicated players rise to the top. Having good items, killing certain bosses means something. New WoW is like driving a fancy car in a rich suburb it doesn't mean much, everyone is rich. Vanilla is like driving a nice car in a poor country, it means you are a hustler and you accomplished something, that deserves respect