Their are two resons for the decline in U.S heavyweights:

1. Most of young kids these days want to fight straight away, they don't want to have to wait 6 months to spar for the first time. They need to learn their trade before they start fighting. This frustrates the kids and they go back to the street which is easier, faster and infinitely more profitable.

2. Some potential fighters quit to try to play in the NFL.

This needs to change if U.S.A are going to have some top class heavies again.