Newer does not equal better.
Another reason would be that the private server scene has a bigger appeal to veteran of the game. People that quit retail at one point or another and want to play the game the way they liked it. So if we go with the history of Blizz's sub base, it peaked at Wotlk. Then the general consensus and as observed by me through friends, many quit at cataclysm. Mop also caused a lot of people to quit and it only went downhill from there. This is why Vanilla, Tbc and Wotlk are always the most popular private server project. Those who play them the most are those who used to play retail in those days and then quit when the game didn't appeal to them anymore. Naturally trying to sell them on the idea of joining a private server hosting the very same expansion that made them quit in the first place isn't exactly going to be much effective.

