I honestly believe this is largely the result of the U.S. performing the vast majority of sorties flown.

Deploying air forces in an expeditionary capability is difficult and expensive. It's much more difficult than deploying a ground unit. Few forces have this capability.

When you're in Afghanistan, and 90% of the CAS sorties are performed by the U.S., then it stands to reason that if an incident occurs, it'll likely have been a U.S. aircraft based on odds alone.