Soccer is a very convenient sport to play. That's one of the reasons why it's so common to see kids playing soccer recreationally (usually organized to some extent). American football requires a specialized ball, and if you play tackle football, you need helmets, pads, etc. Most Americans have played touch football many times, I think. Baseball is now played only with helmets and specialized fields. Hockey requires sticks.
All you need to play soccer is a round ball approximately the size of a soccer ball, and any four markers to mark the goals. Also, everyone is phsyically capable of playing soccer (though most will not be very good), while many people would not be willing to take the pounding of American football, and baseball isn't fun if you suck.
Americans like watching American football because well, there are frequent scores, the battle for field position is visible and tangible (unlike in soccer where a team can dominate and play in the opponent's end of the field, but then be hit with a counterattack if the opponent suddenly gets a surge of energy or a goal kick or whatever, followed by a lucky bounce/touch).
I've watched MLS games from time to time. The level of play/intensity is nothing like the world cup. I would definitely watch soccer (probably casually) if MLS athletes were like that. As of now, the best baseball players in the world play in the US, and so do the best American football players.