A few reasons other than what has been mentioned.
1) Kids don't play outside anymore. Wiffle ball, stick ball, wall ball, touch football, street hockey, riding bikes, etc. How often does that happen now? Video Games, iPhones, Netflix, and lazy parents are killing youth sports. Kids used to get in trouble for breaking neighbor's windows. Now they get in trouble for cracking the iPad screen.
2). Failure. When kids start separating in talent at 9/10/11. Instead of working to get better they quit. With most sports being year round now, they join a spring basketball league, or pick up lacrosse. Or even worse, they do nothing. Growing up, everybody played at least 2 sports. Many played 3. Now some kids don't play any sports. They come home from school and do nothing. Which leads me to #3
3). Poor Parenting. There was another thread recently where a HS player didn't want to play baseball anymore. Not for everybody fine. But unless there is another meaningful activity or a serious interest (school clubs, instrument, school genius, etc) there is no reason to do absolutely nothing. Barring any sort of physical ailment, I don't understand how a kid can not play a sport, not be involved in some sort of club, or be some all A's whiz kid in school. I see more and more kids in my area not playing any sports, but they're not good students either. I blame parenting. If your kid doesn't want to do something, fine. But he can't not do anything. Lazy parenting breeds lazy children.