It's not because of the dearth of scoring (think hockey). Nor is it due to the theatrics of faking injury (think "professional" wrestling). It has nothing to do with the kissing and hugging after a goal (think of all the ass-slapping on the NFL sidelines -- far more expressive of repressed homoerotic tendencies). No, it's the fact that soccer is not a game that can be interrupted every 3 or 4 minutes for commercials about beer and trucks.
When in doubt, follow the money.
A soccer game is composed of two 45-minute periods and a 15 minute half-time rest. In other words, it takes just under 2 hours to watch a complete game, including the convenient 15 minutes to go get a beer, take a piss, call your mom, and so on. An American football game is composed of 4 15-minute periods, plus half-time, plus innumerable commercials. One ostensibly 60-minute game can take 3-4 hours to watch!
That's why soccer isn't popular in the States. Madison Ave. can't figure how to make it work for the money boys, so they don't sell the idea to the punters.