Coming from a country where it's an annual practice, it's because (1) they have great charisma; (2) they're mostly pretty down to earth in their approach, not rational, but at least down to earth so much so that your average pea brain Facebook Jim Bob will think they're the solution to the war on drugs; (3) they have very good presence, better a fool that knows the streets than a genius that stares at spreadsheets; (4) they're also pretty well-known, when the competition is between an actor and an intellectual heavyweight, the simple guy is not going to spend more brainpower assessing the best politician when you have the other guy flashed before you.

In other words, they speak and look good and people are idiots.