Yes for some people it is the subscription cost, or not wanting to support Blizzard, or wanting to re-experience an old version of the game as accurately as possible.

There are also those people that like hopping between realms whenever new ones are launched to get the “fresh” experience and don’t mind losing an old character when they’ve done all they’ve wanted to with it.

I do hope that more of these projects allowed you to export your character data to perhaps use on your own self-run private server. None of the ones I’ve seen allow you to do that.