I suspect it's because historically the best servers for PVE end game content have been PVP servers, so you pick your poison. That said, it's easy to see players consistently roll over to PVP servers with a vast faction imbalance in their favor, and I suspect we'll still see more and more of that in Classic as players get an idea for what the populations are on each server.
So in return for answering your question, I would ask you to explain to me, why do people who want to world PVP roll PVP servers where there's no one on the other side to fight?