I have no proof or anything specific to draw this from, but I always had a suspection some of it was do to the fact that the Vertigo editor(s) simply want to distance themselves from the main DC publishing arms to stay away from people like say, I don't know, Didio, who really have no business being in charge of creative directions. I mean, even know it seems like it can happen, Madame Xanadu's had several cameos and guest spots from main DCUers.
I'm sure that's not the only reason or anything, but with the way Didio's been running DC's main publishing, I can see wanting to shut them out completely from your imprint whenever someone like that comes around. Just a theory on my part, though