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Rugby is called a "hooligan's game played by gentlemen," while football is a "gentlemen's game played by hooligans." Sad to say, but it appears that Americans prefer the hooligans to the gentlemen.
American sports fans and the mass media feed off of controversy and breaking news stories, such as the recent allegations against New York Jets quarterback Brett Favre that he sent inappropriate text messages to an attractive sideline reporter.
This sort of thing doesn't happen in rugby, although it does appear to be happening with increasing frequency in English football (soccer). Players respect each other and whatever happened on the field generally stays on the field. There's never any need to escalate the conflict.
Even as I watched Sale humiliate El Salvador, the visiting team never got angry that the Sharks may have been running up the score. Nor did they give up even when the game was clearly out of reach. In the final minutes of the match El Salvador played hard to keep the Sale score under 100, and they succeeded.
Perhaps American sports fans are simply wired differently. Sports for them is more than just a matter of entertainment, it often becomes a life or death struggle (see any Boston Red Sox fan before 2004). Having a team lose that badly would be considered a form of personal humiliation, and would likely lead to serious physical altercations.
Rugby is somehow too cultured for the U.S. It's an ancient game that has only recently been made into a major international sport. While the U.S. has a national team and several amateur leagues scattered throughout the country, rugby has still yet to emerge in the public consciousness. Maybe it never will.