Ever seen some of the lower level EPL/Serie A/Bundesliga/etc games, or you just tune in for the Real-Man U-Milan types? There's plenty of fans of soccer in the US to support a top level league, unfortunately many of them would rather tune in to support some team thousands of miles away.
Well thats global. I have some favorite NBA teams, i like some Spanish or English soccer teams, even if i am defined Panathinaikos fan.
An other big deference that i see USA sports fan vs European sports fan, is that Americans sees the sports as a pure show. In Europe a soccer team is much more than just an entertainment organization. Almost every team represent either political ideas, or areas etc, or group of people that have common characteristics
On an interview that an English coach had (i forgot his name), they asked him if football is matter life of death something like that he answered, no. Is much more that that. (If someone remembers exactly what happened on that interview please correct me), but i am giving the meaning.
We have seen in last few years 2 teams in NBA that they moved, and the rumors saying that will be more in the future. Is something that you cant do with the historical teams in Europe. Some tries that happened in Greece, those teams died.
Those are some reasons why soccer is so popular in Europe, and maybe they are some reasons why in US doesnt work very well.