American politics has always been shaped by a “Western” mentality. I presume that it goes basic to the “Wild West”, but it has also been fed by Hollywood.
The world is full of goodies and baddies. The baddies outnumber the goodies, so the goodies are always afraid of being overwhelmed. If they are not careful, the baddies will ride into town and shoot the men, steal their horses, burn their houses and kidnap their daughters.
The goodies need a strong sheriff or marshall to beat up the baddies when they attack. They do not give this role to another goodie, because he would not be mean enough to deal with the baddies. It does not matter if this strong man has been a baddie in the past, because only someone mean and tough can to beat up the baddies.