For my two cents worth, In no particular order,a couple things that I think made Vito the leader that he was :

His reputation among the Families for being a man who preferred compromise over violence.
In the Novel,if Vito said that if a man would not listen to reason, it was generally understood by others that an unpleasant resolution might be the next option.

He had probably the widest political influence of all the Families.
He formed and maintained these relationships by staying with the rackets that wouldn't embarrass his Family or the reputations of the officials on his payroll,(gambling,unions,etc) as opposed to that pimp.Tattaglia.

All of the other old school mobsters respected Vito as a man who's word was iron clad,and he was not a man to make frivolous promises to.

He rewarded loyalty,kept a low profile,and in general tried to build lasting relationships with both underworld and upperworld people.

I'm sure there's other factors,but these are the first few that came into my head.