A huge problem with the game catching on in the US is that it doesn't lend itself very well to TV marketing. The US is a country whose programs and broadcasts absolutely thrive on commercials - the average American football game, on TV, is about two hours long and only somewhere in the neighborhood of 11 minutes is actual game time. The rest of that two hour black is dedicated to replays and, not surprisingly, commercials. With there being no time outs in soccer, you're left with a sport that only provides a very short window for advertisements during a 90 minute game.
I'm not even sure this would do anything for the sport, winning often doesn't guarantee popularity. As an example, my favorite sports team, the San Antonio Spurs, are absolutely underrated by the American media. This is a team that has won five championships in 17 years, that regularly finishes in the top four teams during the regular season, that has only finished out of the top five once in 15 seasons, basically the most successful American sports team by winning percentage, and yet they're criminally underrepresented by media here.
Other teams that don't win as much get much more notoriety because their players have these loud and boisterous personalities, which I think is what the broadcasters and the people in general want. I've always felt that soccer doesn't allow for this as regularly as other sports and I think that, among other reasons, does a lot to make it difficult for potential fans to identify with players and find some sort of connection.
But I think the biggest problems is just that advertisers can't shill their products out as often in this game as they can with other popular sports in the US.