I mean, I played Vanilla and raided fairly hardcore and enjoyed it at the time - but in my personal opinion the pinnacle of WoW was TBC.
Many of the annoying aspects of Vanilla were fixed (yes, there were annoying aspects) - the good things like attunements (which I think were great for the game and the raiding community as a whole) were still a thing, both races had all classes which didn't give an edge to any race on certain situations.
The story line was great, the raids were awesome with some of my personal favourite ones like Lady Vashj, Illidan and such... .
I'm not going to point out all the things i loved about TBC but i'm wondering if i'm alone on this.
Maybe it's nostalgia for me and the fact that I made the leap from a fairly hardcore raider to a full on hardcore raider in TBC with all server firsts and a high world rank that makes it special to me personally.
In theory - they could've gone with a few servers for Vanilla only, for TBC and maybe for Wrath - once they figured out the technical difficulties of getting the older version of the game to work on the newer technology I would think it's only down to setting up the relevant game patch on the server.