Wow, ignorance abound. It's all about football in the fall, basketball/hockey in the winter, and baseball in the spring. That's it. That's America.
Soccer's different; hell, we don't even call it by it's real name. There's three professional leagues in the U.S. None of them matter. They put a new stadium/team in my neighborhood a couple years ago. I've never been to a game. Only weirdos who don't play baseball or football play soccer growing up. Notice how most of the USMNT were army brats born outside the U.S. - a place where there's a lack of football/basketball/baseball.
Before 1990, soccer was a weird hobby that rich kids who studied abroad would gloat about. It's grown tenfold. With the concern over football concussions, I only see it expanding. Plus, every sport that's not the NFL is mostly overrated in terms of fans. People in LA couldn't pretend to care about the Stanley Cup. Winter is split with minority (NBA) and whites (hockey). Baseball is dying.
Soccer is great because it's different. Plus, bandwagon fans can go wild. Oh your great-grandmother was from Germany? I'm sure you're a huge fan. Problem with different is that it's only for the younger generations. No oldhead is going to jump into this sport for anything other than 'Go Yanks.'
Long story short, soccer is just another sport that isn't American Football.