A Mafia Don is an absolute monarch. But like all absolute monarchs, he has to constantly exercise his absolute power discretely, and be on guard for conspirators around him. In other words, he can't do whatever he wants to do--if he wants to do things that he knows are patently stupid or that will piss off everyone around him.
To zero in on just two examples:
Joe Bonanno was a true absolute monarch. He wanted to have his son succeed him. He probably could have done that. But he wanted more. He stupidly conspired to whack two fellow Dons: Tommy Lucchese and Carlo Gambino. Of course this plot was revealed, and the Commission tried to strip him of his Donship. The Commission failed to put their own man, Gaspar DiGregorio, in place; but Bonanno was ultimately forced to abdicate, and his son never got to be the Don.
Paul Castellano thought he could violate certain Mob "protocols": he carried on under his own roof with his Colombian housemaid while his wife and daughter were present; did not mingle with his men and required them to visit him; did not pay his respects to his deceased underboss, Neil Dellacroce; and squeezed money from his subordinates unmercifully. All of that was tolerated. But when he moved against Dellacroce's protege, John Gotti, he went over the line. It's not that Gotti was so popular among Castellano's men--it was that they'd had enough; and when Gotti volunteered to take the risk of killing Castellano, they didn't object. That's another way that a Don doesn't have absolute power. For that matter, Gotti never was able to impose his own son as his successor.