User blog comment:MrAnonymous/The Alphas?/@comment-4920092-20120513212035

The basis of the partnership was that they both ate humans, in a manner, so the Leviathan's goal would benefit the Vampires. It was stated in the episode.

ImperiexSeed is right - so far, Leviathans aren't even close to capable of pulling a resurrection off.

And I agree that there are probably more than the Alpha Vampire left, but the boys don't know about them, and they won't be easy to find. Crowley couldn't have captured every single Alpha, and who's to say that, of the ones he did capture, no more escaped like the Alpha Vamp did before Castiel killed them all.