He's proved an ability to grasp complex issues through turning around New York City and also how he'd deal in an extreme crisis. Quite a long track record. Sure, he's been clearly wrong about some issues, but on the whole he's been quite good.
Rudy is a case of someone who is book smart but dumb when it comes to common sense. How could one not grasp the concept of "shall not be infringed"? A kindergarten student could understand that.