They’re all different versions of the game offering differing styles of gameplay. Retail is endgame only. The Classics tend to be a mix of old guard like me who think MMO means leveling alts, and other people that just want to raid log.
WoW started as an idea of a playground with a bunch of different activities people could engage with. It turned out that WoW gamers are more niche players than generalists; they find the playtype they like and they stick almost exclusively with that.
Now the one world playground is segregating into differing playstyle niches.