I say this as a fan of a ton of different sports and someone that played soccer almost his entire life. Some of my earliest memories of sport involve playing soccer and some of the highlights of my youth involve moments of glory on the soccer field. I have two false teeth because of a soccer injury involving my face and a goalpost (saved the goal) and I got a scholarship to college based on me being considered very very good at the sport. I live in a city that has a professional soccer team as well. I have never been to one of their games despite the fact that they basically give the tickets away.
So if I, a lifelong lover of playing the sport, isn't going to watch it, why doesn't the rest of the population do so?
I've heard stories about big clubs in Europe such as Liverpool and Manchester United where it is almost impossible to get a ticket to go to a game and if you are lucky enough to find a ticket it is going to cost you hundreds of dollars for the opportunity. The games here in my own city are so sparsely attended that the almost always close the upper decks and only allow people to sit in the front rows - presumably so that they don't have to clean the top part later and so the TV crews can make the games appear better attended than they really are.
3 million people live in the metro area yet we can only get a few hundred people at these games despite being one of the best teams in the league
While we do understand that the rest of the world absolutely loves this sport, it simply doesn't resonate with the average American just like "our" sports like American football and baseball are largely ignored by the rest of the world.
Once every 4 years Americans take a cursory interest in the sport if the US manages to make it to the world cup but it is kind of funny how as soon as the United States is eliminated from it - and they always are - viewership plummets. The "fans" don't care about the sport, they are simply patriotic and want to be number one at everything even if they don't like it.
I watched with some interest in whatever year it was that the States managed to emerge from the "group of death" and make the brackets only to get eliminated by Belgium. This was where goalkeeper Tim Howard basically took on the entire Belgian team by himself but one eventually got past him. It was after this that nearly all Americans stopped watching the event altogether. I don't even remember who won that year, was it Germany?
Soccer is one of the most participated in sports among youth but even though all kids end up playing the game, this doesn't end up turning them into fans of the sport later. MLS ratings and game attendance is pretty poor and many of the teams don't even have dedicated stadiums. It was funny in the past when the league cared so little for the sport that games would often be played on surfaces where the American football yard markings or the dirt from the baseball field was on the pitch.
I think the main reason why Americans don't take to this sport is because we demand scoring and we absolutely despise ties / draws. When a sport can be played for an hour and half plus that magical seemingly random amount of extra time and the game is still zero to zero, this is a great way to lose the interest of the average US citizen. I've been told by my non-American friends that the tension that is created by near goals is a big part of the reason why they watch and enjoy it, but this is something that just doesn't exist for most Americans. We want real goals, and normally a lot of them. This is why we like American football and basketball: High scores are almost guaranteed.
The second major factor that I think turns Americans away from watching soccer is the faked injuries.
These attempts at getting free kicks are just annoying to us and something that we won't tolerate even if it is being done by our own home team. If you were to ask most Americans if there should be severe penalties for faking injuries in any sport most would say "yes" and this is probably something that could be prevented globally with some sort of review process yet it isn't done and I really have to wonder why that is. How many goals have been scored on the backs of faked injuries?
I'm sure there are other reasons that Americans don't watch this sport such as being far more interested in other sports but I think these two reasons are the main ones. I honestly don't see this ever changing either.