First, let me say that I really can't bring myself to give dang whether the USA team does well in that World Fly Fishing Championship or whether Team Hyde or Team Clackacraft wins the Masters. I'm not even sure making fly fishing competitive is good for the sport.
Nonetheless, I have observed this about fly fishing competitions in general: They don't seem to truly be general fly fishing competitions, but rather trout fly fishing competitions or casting competitions.
A competition that truly tests the competitors' abilities at fly fishing in general would have to include warmwater and saltwater components as well as trout and salmon.
I bet the Europeans wouldn't have quite such a lock on the gold if the World Championship were truly a fly fishing world championship rather than the world championship of European-style trout fishing.