While soccer has never been one of the big sports in the US, it has continually increased in popularity the past 20 years (coinciding with the start of MLS). I think what the owners are betting on is the generational shift that will make it popular. My nephews, and most of their friends, watch MLS and know the top players, as well as top international players. Also, the amount of kids playing soccer now continues to grow and is catching up easily to baseball or football. This is something that was missing from most of our childhoods (unless you grew up in Europe or had European parents), but is becoming a part of the new generations' lives, just as football and baseball was to us. In 2000 USYS participation alone busted 3million and has been above that every year.
The main challenge, of course, is advertising. 45 minutes without advertising isn't a big money-maker. I think as popularity grows, though, we'll start getting on screen ads in the corner or other ways of incorporating ads into the games which will help make it more profitable to broadcast. Advances in camera technology also make it more exciting visually. Not just one or two fixed camera angles for 90 minutes.
Oh, and I read an article saying over 1 billion people will watch the final match of world cup.