I found this excerpt to be the most interesting and revealing part:

5. Lack of Incentive: The profit motive
"Because it is centrally run, MLS clubs don't have the incentive of a cash windfall for successfully developing young players," Agoos said. The situation is exacerbated by U.S. labor laws that prevent the prospects from signing contracts. "The players can move anywhere until they are 18, which is an additional deterrent to serious development," Lepore said. "Profit is inherent in South American systems like Brazil or Argentina and so their clubs scout aggressively. In MLS that process is just beginning."