I read an interesting article last year on why there's such a dearth of American heavyweight contenders:
The obvious reason is that Americans just aren't hungry enough to put in the pain to be contenders--but the former Soviets, Mexicans, etc., are. But another reason is that there's a tremendous emphasis in all sports on individuals setting records. So, any adolescent or teenager who's big, muscular and shows some athletic prowess is going to be steered into baseball, soccer, hockey, or other team sports that formerly didn't necessarily require players to be physically huge and muscular. Everyone wants new records. That's one of the reasons why steroid doping is such a problem across sports.

What's more, only a small handful of boxers get far enough to make real money, and even the champs get the piss whaled out of them over time. But a team guy can be fair to middling and still have a reasonably long, well-paid career--and keep his brains from being scrambled.