Just a guess: the A-10 is designed as a support aircraft for ground troops, which means that if it does its job perfectly, the Army gets to claim a decisive victory. The F-15 is an air-superiority fighter, which means that if it does its job perfectly, the Air Force claims a decisive victory. Inter-service rivalry is a Thing...from the POV of an ordinary citizen, it doesn't matter as long as we whup the bad guys, but from the POV of a career military bureaucrat it matters a lot whether the victory is credited to his branch of the service.