Why did Michael Corleone fail where his father succeeded?

There are two themes in the Godfather movies, Respect and Nobility.

When Vito comes to America, his rise to Don is seen as noble, he is providing for this family. When he becomes Don it is respect, he does things for people and they do things for him. At Connie's wedding we see them lining up with genuine respect and nervousness.

The Corlenone family is also respectful, they have peace between the five families, no narcotics (and no mention of the sex trade), mainly booze and gambling, things that at that time were more acceptable.

The trouble only erupts when Vito's moral stance prevents a move into the grey area, and Sonny's stance makes the other families move to get someone more sympathetic to their less noble ideas.

Michael has (despite his war record and unwillingness to get involved) none of this nobility. He breaks the sanctity of the family meetings as an assassin, and once Don his first act is ignoble as he tries to remove the competition in one go.

He has also has none of the respect Vito had, his own brother betrays him, the political allies allow him to be dragged to hearings, his moves to legitimise his businesses are betrayed.

Once he tries to move completely out of the crime world, he has no respect from the church (despite massive donations) and just becomes a target for an even worse version of him.

So it's not surprising he fails, where Vito was noble, Michael is ruthless, where Vito has respect, Michael has fear, and his world crumbles due to it.

You really can't compare Vito and Michael's tenures. First of all, Vito's rise to power was pretty quick; he took out one guy who apparently had no connections or backup. Michael, on the other hand, grew up in a time of family power, each of which had plenty of "foot soldiers" who could enforce a family's will. In Michael's time, Vito would have been hunted down and his whole family killed, most likely while Vito watched, and then Vito would have been dealt with.

Secondly, corruption was much more widespread in Vito's time -- or at least it was more out in the open. This means that Vito had control from every level once he was established, whereas Michael was dealing with the tail end of Organized Crime as Vito knew it. Two very different eras.

Lastly, Godfather III is a rather detailed story. There is so much backstabbing, bickering, dealing, double-dealing and deception going on, it's no wonder Michael loses everything.

• You've got Michael trying to legitimize,
• you've got Zaza trying to illegitimize,
• you've got Michael trying to make a deal with the Catholic church,
• The other families want in on the deal,
• Zaza plots to kill Michael,
• etc...

And it all comes crashing down when the Pope gets poisoned and the church renegs on the deal. It's a mountain of problems that I don't even think Vito could have successfully unwound. And even in all this, the only physical casualty is Mary.

I'll close with this: one other huge factor was that Vito protected his family. His wife never asked questions, and (as an Italian I can attest) back in those days divorce wasn't an option. Family was everything. Michael's divorce split his family, and in the end he had nothing. He died alone.