so if the current market was a little better, and there was more ammo available, why would somebody go for one vs. the other?
If wishes were fishes you mean?
Well typically a pistol chambered carbine itself is a lot less money than say an AR (though there are some very expensive ones too).
Take the Keltec for example. If I bought the Glock version and was myself a Glock pistol owner, wa la. I don't have to buy a bunch new mags and I have a carbine good at medium distance. The added velocity you get out of that MUCH longer barrel means that puny 9mm hits with some authority. There was a time too that pistol ammo was a lot easier to find than rifle ammo, and still is for a lot of folks. That's another part of the appeal. That and now instead of needing a rifle caliber and a pistol caliber all you need to do is shop/reload for one caliber. That's kind nice.
And how about pistol chambered carbines can just be fun? Sometimes it's okay to just have fun when shooting. You don't need to be tactical 24/7.