Because of the connotations. All of which do not befit president of the United States. It's called dignity. It doesn't matter if you wear your hat like a teenager but it does when Obama does.
A few years ago, I was in a mall at Christmas time. I saw a group of four black teenage boys wearing hats (backward) with price tags still on them. I walked over to a security guard and told him that those boys shoplifted hats and are wearing them. He smiled, and told me “Ma’am, that’s how the kids wear them now.” I said, “You’re kidding, right?”
“My bad” — as they are saying now. I’m down with it, though....for sure.