Why United States doesn't dominate soccer?
Because soccer is not popular, it doesn't make you as much money or respect as other sports. Strong athletes will overwhelmingly choose sports better for them in the long run.
Elitist. For some reason, in the US, soccer is expensive. Leagues are expensive, classes are expensive, etc. It is squarely an middle to upper middle class sport. This cuts off a huge part of the population. I cannot tell you why because everywhere else, soccer is for everyone and a lot of famous players come from poor backgrounds. When the game was invented, it was a working class' game.
Soccer starts too late. There is no progression from childhood to pro like there is everywhere. The best clubs and teams of the world (and even the ones that aren't so good) have special programs for K-12 kids (e.g. Barcelona has been training Messi since he was 13). Most soccer players I know didn't start playing seriously until high school or even college.
Or go to a park and practice ball skills, solo. Ex-England captain Wayne Rooney was famous for having done that for hours at a time as a kid.
I'm not actually a particular soccer fan and quite enjoy basketball, but it's definitely not true that soccer has higher kit, space or numbers requirements to get in a game as a kid.
This is what we did. A great way to spend summer when you are teen.
As for practicing ball skills, the very nickname Pele was from the sound of the can Pele played instead of ball - his family was too poor to have soccer ball.
Over here (Uruguay) there's a whole cottage industry of five-a-side soccer fields, many workers play it after office hours.
Edit: as a side comment said, poorer people usually cordon off a stretch of road and play there, that's also very very common (especially during the day when there's no much traffic). Two rocks (or logs, or bundles of clothes) can serve as goalposts.
I was only ever a mediocre player, mainly relying on speed - which also got me in just about every school team (small high-school, small market town). At Uni our team did come 2nd in the intra-University cup.
Why did I do all that rather than basketball (a sport I've really enjoyed playing), pervading culture would probably be the main reason (which in turn means availability of fields/courts, etc.).
After Uni I took up volleyball because that was the social sport played at my workplace; now I do karate because that's the sport my kids are doing.
tl;dr You can play football without a football and with 1+ players.
1) Latino immigration and their inherent interest in soccer
2) Pigskin Football is plummeting due to concussion fears
3) Steady rise in popularity of the sport, especially as a social/hipster rebellion to old guard leagues like the NFL
4) MLS, for better or worse, seems stable and can provide some degree of professional support for players. That will attract more athletes and keep athletes in the sport
I think the US will rise to the level of the less talented Euro nations that qualify, like maybe Denmark or Iceland.
Pay to play: for youth development, the player's family has to be fairly well off. This is pay for either an academy, or going to a D1 college team, paying for travel, etc etc
Limited coaching talent: not very many soccer coaches play or even watch soccer regularly. This leads to an emphasis on agility and strength rather than skills and strategy. Agility and strength work out pretty well if the team you're facing has the same focus, but if you're playing against a team that has drilled and nurtured intelligence it will end badly for you.