Because they weren't a good team. Period.
I must have heard at least ten times yesterday that they were robbed from a chance to advance in this World Cup. That's just ridiculous. I would venture to argue that the US lost because they had fewer shots on goal than most other teams in this first round. I know that's a bold statement, but I'm willing to step out on a limb here. The US played the game just like they did in 1998.
Although I love Robinho and would be ecstatic to see him get more field time, but Ronaldo showed that even a fat man can run the game.
Now, the question is... Can France and Zidane pull through today?