Supercross has been the American focus since at least the early 80's, but in those days it was an asset. Supercross races are basically sprints, and American riders caried that mentality into motocross races. Back in the early 80's Europeans treated motocross races more like endurance races, so when American riders showed up giving it 100% from the moment the gate dropped, it caught it them off guard. Of course, the Europeans finally caught on, but by that point the Americans had such a deep pool of riders to pick from it made it hard for the smaller European countries to find three riders of sufficient skill to be a real threat.
While Supercross has continued to grow in popularity in the US, motocross has never really expanded beyond it's hardcore fanbase. Meaning that more and more emphasis has been placed on riders doing well in the former, to the point that Supercross specialists, like Brayton, have had an easier time finding rides than motocross specialists, like Nicoletti.
This problem is compounded by the fact that Supercross and Motocross have become so different that they are hardly recognizable as the same sport. In other words, Supercross has become a liability at events like des Nations.