There's a decent video of someone playing WoW Classic for the first time - https://youtu.be/NjQgoaagS-E

And it basically came down to "classic WoW was simpler and for new players provided something that modern WoW doesn't/can't really - especially the spontaneous community.

WoW started out as an MMORP, but it's really a massively single player game until you're top level anymore.

The demand for classic wow got much, much stronger after cataclysm, because then you couldn't even "pretend" anymore.