Re: Genuine curiosity
It's pretty simple, really; politics in America tend to be brutally polarized. There are only two parties, really, so instead of dozens of parties blustering for minor control, there are two parties who stand toe to toe and yell at each other.
Since the two parties are almost always on opposite sides of any issue, voting preference comes down to voting for or against something. If you vote for "cut taxes and limit government spending," then anyone who votes opposite is, obviously, a wasteful big-government socialist, intent on destroying small businesses and the American Dream. If you vote "free healthcare for everyone," anyone who votes opposite you is, again obviously, an inhuman monster who would rather watch grandma die than pay a few measly dollars more in taxes.
If you take that one step further, and give these polarized issues to people who don't understand them (read: the vast majority, on both sides), you end up with a huge crowd of people that believe the other side steals from the poor, eats babies, and drives the wrong way on a one way street. There is no room for logical argument, since few on either side understand their own stance, never mind the opposite stance. The few that do understand are fed up with arguing with idiots and would rather just unfriend you for believing differently than themselves anyway.