Another reson could be that Swarm was the first dive drop, so B&M used it to discover a better way of doing it?
Anyway, I think that the only reason why wing coasters have dive drop and others don't is, because of the same benefits of a heartline roll. Swarm, X-Flight and the one in China all have inline twists. So, the movement of being in a wing train going through the twist is almost like a mini-drop. Whethers, other coaster got heartline rolls, because having your head force downwards at a very high speed would apparently cause people to faint.
Correct me if I'm wrong, I'm only judging this on the shape of the trains like bob_3_ said and the shape of the twist.