Actually, that title is a bit of a misnomer. Air-powered weapons never really “went” anywhere to return from. Once a real alternative to early matchlock firearms, air-powered weapons had mostly become toys, tools for hunting and weapons for marksmanship competitions.
Once society broke apart however, those with technical know-how began to take these weapons much more seriously. Gunpowder is not always easy to come by, especially if the nature of the apocalypse is something that destroyed most manufacturing centers (such as a nuclear war).
In short, the main advantage of these weapons isn’t so much in their destructive potential (even though a few of these weapons pack plenty of that) but in their ability to make the wielder self-sufficient. A smoothbore air-powered firearm needs nothing more than air and something hard and relatively round. In a pinch this could be a small rock but with a proper bullet mold, a survivalist could sit around the fire at night and make the bullets he’d need tomorrow for hunting or defense out of any handy soft metal.
This is exactly the reason why Lewis and Clark carried at least one air-powered rifle on their trek across an untamed America and in the wake of the apocalypse new pathfinders have followed in their wake.