Drezner captures it:
There are plenty of sports in the United States that occasionally capture the intermittent attention of the casual sports fan, but won't "break through" the sports zeitgeist until and unless the United States fields a successful national team. This is how it tends to work with the Olympic team sports, and it's how it will work with the World Cup. If the United States can advance far in this tournament, Americans will become more interested; if not, they'll switch back to baseball and the NFL draft. In this approach, the casual sports fan is using a strategy of "rational ignorance" -- i.e., not caring until the team is sufficiently successful.