I don't think Hagen was quite leadership material (no knock on Hagen by definition most people aren't) and just as importantly I don't think even if Hagen had been well suited for the job that he would have been accepted by the other Family members, not to mention the other Dons.
Furthermore, no one who wasn't Italian could become a Mafia don (except on an informal temporary basis, like Michael did with Tom in GFII). Non-Italians can be consiglieres and associates of Mafia families (like "Jimmy The Gent" Burke was to the Lucchese family in real life, for example), but not made members and certainly not dons.
