In "Sympathy for the Devil" [Spoilers] ..

The way that I understand it they have to have a specific genetic make up to be a vessal. And each angel requires specific genetics. Like how Cas could use the father and the daughter, their genetics would be similar. Also, door calling it the sword of Micheal that would keep the demons from knowing what exactly it was. Dean has always been the vessel of Micheal, there isn't anything that changed that, that is meer than likely why he was pulled from hell too. He is the last remaining vessel for him. Dad is dead and Sam is polluted, that leaves Dean/Micheals last hope. I really don't like the angels of the demons. The one thing I will give demons is that they are honest and doing what their leader wants, while the angels are running rampit and not listening to their lead.