I think for a lot of people it isn't about how good the encounters are or how the raid was designed but more about how they experienced raiding and WotLK was the most pleasing experience for most people.
Vanilla was OK. Very simple fights and the real problem was just to get the right gear with the right resistances for enough people and get enough people. BC was good but again encounters were simple and only some encounters stand out.
WotLK was special because you had both, 10 and 25 man difficulty with 10 man being significantly easier than 25 man. That allowed you to pug 10 man raids and everyone could start raiding 10 man. I remember how tradechat was flooded with people looking for raids and it took 10 minutes at most to find a group. Back then it was harder to find a dungeon group.
Cata was OK but with it destroyed pug raids and with MoP and LFR it's the same. Sure the encounters in Cata and MoP are better and all but raids in general are less accessible to people as back in WotLK because you need more time to find a group via LFR.