Three things:
1. Harkon mentions it in the Dawnguard quest itself. To paraphrase here after you tell him your a werewolf
Spoiler "Mmm, yes, I could smell it. Doesn't matter, my vampire blood will be able to remove it and turn you into uber vampire."
.
He's essentially say the two diseases can not mix and one will push out the other.
2. Werecreatures can't get diseases.
3. Remember that lycanthropy and vampirism are Daedric/Divine blessings and curses. Hircine calls his werecreatures his loyal subjects. Meaning that once someone turns into a lycanthrope, they are bound to Hircine. He wouldn't want his subjects joining Molag Bal and his vampires. And Molag wouldn't want werewolves among his blessed vampires.