Originally posted by Double-J:
Perhaps my question should be then why didn't Vito want to give Carlo a chance? 
It's not explained too well in the movie, but the book talks about how the Don made a check into his background in Nevada and found his character lacking. He didn't fight Connie about the marriage because it's what she wanted and it's just his way to let his children figure out things for themselves, but he didn't trust him enough to give him enough power to do serious damage to the "Family".
