It's pretty simple. The reason Americans can't grasp the hold soccer has on the rest of the world actually has little to do with how good or bad of a sport it might be.
#1: Most other countries don't have multiple sports the way the U.S. does. They might have a freaky small regional sport, like cricket in England. But other countries don't have large nationally-televised big-money sports the way the U.S. does with football, baseball, basketball, hockey, and even golf. So, most other countries have become fiercely attached to soccer, especially their national team. Which is the other piece of the puzzle, because....
#2: Most other countries tend to be pretty damn small and pretty damn packed close together. As a result, the entire national identity has become wrapped up in their soccer team. In the U.S., we don't have densely-packed countries like Europe or South America or Africa; we have states, and the states don't have nearly the sense of cultural independence from each other as say, France from Spain and Spain from England and whatnot. Soccer, for the rest of the world, involves national pride and national identity. When the World Cup rolls around, it's like the Olympics.
For these two reasons, soccer will always be the biggest thing outside of the U.S., and for the same reasons soccer will never become anything here in the States. Unless the other major sports all fold, and each state in the Union creates a team, and we have a 50-team U.S. league. But even then, without the cultural differences, I doubt that a "Kansas vs. Oklahoma" soccer match would cause enough passsion for there to be rioting and tear gas needed.
When you get down into a specific country, sure, their games are more like our rivalries. When Madrid plays Barcelona for example, or when Chelsea plays Manchester United, those games are more akin to what we're used to. But, even then, U.S. city rivalries in a sport rarely reach the levels of dislike generated by these soccer matches. Some college football and college basketball games might, as well as some big rivalries (Red Sox and Yankees, for one).