Money
If it pays well soccer will attract good athletes
I didn't grow up with soccer so I don't have a feel for the intrinsic fun of playing soccer vs. football or basketball, which I did play. I came to soccer as an adult while watching a daughter become a reasonably competitive (not elite) player. I've watched enough to now believe that soccer is probably the most difficult of sports to play really well, mainly because you have to handle the ball and shoot with your feet at the same time that you are running and cutting. Baseball fans often say that hitting a baseball thrown by a major league pitcher is the hardest feat in sports, and for sheer quickness and hand-eye coordination, maybe it is. What soccer players do is akin to hitting a 100 mph fastball with a club attached to your feet while sprinting down a basepath, stopping and cutting as necessary to dodge defenders. And of course, it's the hardest of all sports in which to score, which is probably a weakness in the eyes of teenage boys oriented to quick gratification.
Soccer obviously has appeal for people of average size, who have virtually no hope of excelling in sports dominated by the very tall and the very big. You could be the best athlete in the world and be 5'10" and 150 pounds, and you won't make it to elite levels in football or basketball. But you can play soccer or baseball. They're all great sports. We tend to appreciate the sports we played as kids. Soccer will continue to grow in the U.S. because it is increasingly the gateway team sport for little kids. Given the thuggery that has gotten out of control in the NBA and NFL, and the risk of injury in the NFL, I suspect that more and more young athletes will stay with soccer. Lacrosse is growing for some of the same reasons.
In general, I think that all sports are best in the simplest form -- the "natural" form as played spontaneously by kids when they go out unsupervised, just to play for fun. MLB, the NFL and the NBA have prostituted their games for television. I appreciate soccer for holding to the true form of the sport. I wish MLB, the NBA and the NFL would revert to the classic versions of their games, but they won't as long as they sell out to television.