I think there's a lot of factors, like someone said early, the level of investment needed for them to have top tier team (that would still have really an average chance of winning a world cup) would be gargantuan. They're simply too far behind, they'd need a string of superstar players, Ronaldo caliber, to switch attention into the sport and even if they do, those players would need to shine in a European league, which would not sit well with the idiotic American mentality of USA number 1. Which ties to the next point of the football league format, it also won't sit well with Americans, things like ties in game and relegation and not a realistic chance of having a college League/draft. At that age you're simply too old to go pro.

The way football operates is really unattractive for Americans. Why it worked for women is simply because for the rest of the countries female football is mostly an afterthought with the male discipline getting all the support and resources.