The answer has its roots in American Colleges and baseball.
‘Football’ has been around an awfully long time. In middle-ages Europe, it often was played by having the people from one town meet the people from another town at a spot equidistant from both, then attempt to kick a ball into one or the other town square. The Italian version of the sport was known as ‘calcio.’ England had a version as well.
In the early 19th Century, ‘muscular Christianity’ was a concept introduced into English public schools, designed to help educate young gentlemen through athletics. Among the sports utilized was football.
Unfortunatly, the schools had troubles agreeing to rules for the game. Some schools wanted to allow the ball to be carried and the carrier ‘tackled’ by physically restraining him and putting him on the ground. Others wanted to leave the game primarily a game of the feet, without being able to grab a player to stop him. Of the former, the most famous school was Rugby, and that version came to be known as Rugby Football. The other version was developed into the Football Association, and came to be known as Assoc. Football, or, eventually, Soccer.
Unfortunatly, soccer became quite popular with the masses, making it ‘unsuitable’ for the young gentlemen who attended such bastions of learning as Cambridge or Oxford. Those schools adopted football programs based on Rugby rules.
Jump to the US. The time is the turn of the century. Lots of people are here from England and other countries where soccer is getting quite popular. It becomes popular here, too, though never quite to the degree it did in places the English held a large degree of commercial sway (e.g. Argentina, Italy, Portugal, etc.). However, the top colleges, among them the Ivy League schools, decide that they have to be as much like ‘Oxbridge’ as they can. Therefore, they develop football based on rules similar to Rugby rules, with some peculiar variations thought up over here. The result, of course, is American, or ‘gridiron’ football. As that game becomes increasingly popular at the college level (remember that in the 20’s and 30’s the college game was what people followed), soccer’s popularity wanes. In 1930, the US makes the semi-finals, losing to Argentina 6-1. By 1950, the US only was able to win one game, and then didn’t make the finals again until 1990.
Lest you think soccer was never popular here, the American Soccer League in the 20’s and 30’s played to crowds of 6,000 or more, better than the original professional American Football crowds.
How does baseball fit in? Well, frankly, baseball makes soccer unnecessary. In England, and in countries England traded with, there was no single sport that had captured the interest of the population. In the US, however, we already had baseball, which used up quite a bit of our sporting effort. In Italy, soccer became virtually the only sport worth patronizing; in America, baseball made development of fan appreciation less easy.
For those of you with interest in sources of history of the sport in America, and abroad, you can read such books as ‘Twenty-Two Foreigners in Funny Shorts’ by Pete Davies, or go to sites like http://www.soccerspot.com/soccerhistory/ . The latter has a series of fun links to articles about the development of the game in the US.