I don't think it's exactly the violence of the sport that's the biggest problem. Soccer can certainly be every bit as violent as basketball or baseball. You try to dribble the ball by me where I can get a slide tackle in, and I'll show you some "violence."
I think the biggest problem with "American" fans is that a large percentage simply don't understand the game, and thus don't appreciate the many subtle things that happen during a game. They don't see the beauty of a well-placed pass, a great run, or a perfectly timed tackle. They don't understand how amazing a 0-0 game can be.
MLS has made a lot of inroads into capturing some fan base. Hopefully it will continue, and educating the fans about the game is important. Soccer is a very popular game with youth, for a lot of reasons, and as the number of US soccer playing youth continues to increase, these players will become adult soccer fans, and I think we will, in time, see a greater demand for soccer in the US.