Me, personally I'm here for the social aspect and the social aspect only. But, from a research/paper for class, this might be an interesting perspective.
1) For some, I think there is a sense of "unfinished business" here. From what I can tell, there are many people that are in their mid 20's. That makes them low to mid teens when they started playing Vanilla. Chances are, they didn't know as much. They're now back with a better understanding of the game to kick some butt.
2) A sense of superiority. There are things that Blizzard got right in WoD. Not many, but some. The mythic raiding content is getting more and more complex. Getting carried for epics is no longer an option. If you're not carrying your weight, you're going to get exposed. Same goes for PvP. Teams will be playing others of their level. No more squashing terrible PuGs. Again, if you're on a rated team/arena and don't carry your weight, you'll be exposed. Some people don't like this. So what do they do? They go to a game where the skill is less, but it's one where they may know better than others.
Now, I don't think this is the majority. I believe most of us are back because of the sense of accomplishment, the community, or just never having experienced Vanilla wow. But, just saying. The above could be contributing factors also.