I was thinking about this again
Michael: It's a bloodline.
Dean: A bloodline?
Michael: Stretching back to Cain and Abel. It's in your blood, your father's blood, your family's blood.
and okay, it occurred to me... Now, I Know that Kripke and his stable of writers haven't followed any theology canonically, but if the bloodline stretches back to Cain and Abel, then it (whatever it is - midichlorians?) is in every human being on the planet, not only the Winchesters.
But then we know Michael was questionable in his interpretation of what we poor mortals know of theology (Really? He "raised" Lucifer? Made him eat his vegetables and tucked him into bed and held his hand as he walked him to choir practice? Or whatever the angelic equivalent is.)
So I guess we're going to have to go along with Amy's interpretation of Gabriel's interpretation of the situation:
the reason Sam and Dean are the *true* vessels is because they're brothers, because they're mirroring the conflict between Michael and Lucifer. Destiny, or whatever, arranged it so that they're the ones who are going to play out the battle best emotionally, I guess?