Money.

Typical amount of commercials time per game:

    NFL                          60-65 min
    NBA                          40-50 min
    MLB                          40-50 min
    NHL                          25-35 min
    MLS/Premier League/World Cup 10-20 min

I was going to go with money -- the amount of money to put kids through soccer in a competitive program in USA/CANADA is way more than the competitive programs in europe. That immediately drops down the amount of talent that can make it through the system + anyone who is particularly good and has any connections to europe goes there to level up faster by playing more competitive players.