You know, since I was in high school (which was a while ago) people have been saying that soccer is going to take over America like it has other countries. It hasn't happened yet. I do not expect it will happen in my lifetime.
Men's soccer is played in the fall in the US, which puts it in direct competition with football. Not only in competition for fans, but for players as well. If you are an American high school, would you rather field a football team that will sell 5,000 tickets every home game in the fall, or a soccer team that a handful of people will show up to watch? Big schools can have both, but most have to choose. Football is where the money and glory are at. It's not a very fair fight for soccer. The only way I see soccer gaining popularity is if football is banned or something. Even then, there's always Rugby.
As for international soccer, I just don't know that most Americans get it. The games are long, nothing seems to happen for forever on end, and there really isn't much national pride in our team. During the Olympics that will probably change, briefly, but otherwise most people just don't care about American soccer.