I think it really depends where you live in the states. Soccer has a pretty cult-like following in pockets of the states, but you are certainly correct it's not nearly as popular as other professional sports.
Personally, I think it all comes down to money. Advertisers, and by extension the major networks, look at a game that only has one commercial break, which also happens to be halftime, in a two-hour timespan and wonder how in the world they could make any money off it.
It's actually kind of sad. I love all major sports and have been to a dozen or more professional games of every major sport. And to me, soccer, by far, is the most exciting sport to watch live.