At that time scarfo was the only made guy in ac so that would make him a "defacto" capo, and I believe he always had ambitions of being boos so I think he wanted to stay close to the power

"McGurn likes you, so I make you. So you are now one of us, if you fuck up, we take it out on McGurn. He is your sponsor. Fuck up, it's his ass. You work in his crew, he is your capo."