Three reasons that soccer will never become a major sport to watch in America:
1. It will never become the passionate pro sport that it is is other countries, because it is not a spectacular/exciting pro sport to watch on TV, unless you are a real soccer fan.
2. It is still primarily considered a social/fun sport at the youth levels compared to the competitiveness of basketball, baseball, football. Certainly there are some levels of competitive youth soccer, but it is not the majority of the sport.
3. It is not a moneymaker in America.
http://money.cnn.com/2006/02/01/commentary/mediabiz/index.htm?cnn=yes