Back when the AF was created, the assumption was that any large scale conventional war would be with the Soviets. That would quickly evolve into either a theater nuclear war or a all-out ICBM exchange. Thus, Pre Vietnam, SAC was the dominate culture of the USAF.

The A-1E was probably the best CAS aircraft ever built: Gobs of payload, 12 hr loiter time, etc. Totally kicked ass, but, alas it had a prop. The AF wanted jets. Fast, shiny, jets. (Still do.)

Intro the A-10. The A-10 was designed to kill Soviet tanks in Europe—kind of a sideline to the "core" USAF mission (Deterrence), but might slow them down until things eventually escalated into a nuclear exchange. Since things would always digress into nukes, and since A-10s were expensive to operate (read not fast or shiny) , and since the army was largely irrelevant…might as well get rid of the Hog. Replace it with shiny fast jet (F-16s) who could give lip-service to a CAS mission, that they’ll never really ever have to do…

Enter Desert Storm. Whoops. A-10 kicked ass. Turns out the army really isn’t irrelevant, the Soviets aren’t a huge threat anymore, every conflict won’t end up in a nuclear exchange, and the USAF is back to ultimately being a support organization for the grunts on the ground. Hmmm. Maybe we’ll keep ‘em around a while….