Tempi cambi. Times change. Vito acted under completely different circumstances. Vito lived in an Italian community where people knew each other and had personal contact. This may have changed already when he moved to Long Island, although the old friendships and relationships continued. (I don't think that Enzo and Bonasera were from Long Island.) Anyway, in the early 20th century personal neighborhood relations meant a lot in New York.
Now, Michael lived in Nevada. There practically was no Italian neighborhood. In fact, there was no neighborhood at all.
This doesn't mean he doesn't do favors. For example, he donates for the local university.