to be honest, I think giving the WC to the US would be a very good decision by FIFA because (as mentioned above) the '94 one was, by a fairly large margin, the most successful WC ever, both financially and attendance-wise. Also, I would dare-say it was primarily responsible for the tremendous surge in the popularity of soccer (yeah I know I committed the unpardonable sin there....) in this country in recent years. Yeah, we don't have any pro leagues to speak of, but pretty much all schools and colleges have soccer teams now and a lot (and I mean a LOT) of people play in them.
However, competing with MLB, the NBA. the NHL, NCAA football and basketball, and especially the NFL is not an easy task for a sport that Americans like to play, but generally don't like to watch. I think this is the main problem soccer has in the US. The vast majority of the population (myself included) think it is incredibly boring to watch, even if they like to play it. My pet theory for this is the low scoring, the lack of any intermediate goals or 'stepping stones' to scoring, and the lack of physical contact. For example, basketball and football are relatively fast-paced and (most of the time) feature a large amount of scoring. Both football and hockey have a lot of physical contact, and baseball and football have tangible intermediate steps to scoring (baseball's bases and football's 'downs' system).
Not a good theory perhaps, but violence, action, and/or the sense of accomplishing something definitely hold an American's attention better than a bunch of guys kicking a ball around and (seemingly) accomplishing nothing. Also, it probably doesn't help that soccer doesn't have any good equivalents to basketball's slam dunks, baseball's crack of the bat, hockey's slams (and fights...), and football's 80-yard (77-meters) passes and devastating hits, though the occasional soccer breakaway can be somewhat exciting.