Several flaws in your basic premise, amprecon:
1. If a carbine is made in .30 Carbine, it is by definition NOT a pistol caliber carbine, because it is in a
carbine caliber.
2. Winchester, Marlin, Henry, and all the repros of them, have been making .357 Mag. pistol caliber carbines for many years. They all made them in .44 Mag., and .45 LC, also pistol calibers. Ruger even made a semi-auto in .44 Magnum.
3. Not so much a heresy, but you must consider the rifle and round that inspired the M1 Carbine, the .351 Winchester Self-Loader, wasn't very popular either. A commercial flop, it was foisted off on the LE community, even then it mostly went to prison guards.
Most people buying any new carbine in .30 Carbine (that isn't an M1 Carbine repro) would be buying it for the nostalgia of the round, and there just isn't the market for it, as slow sales of the Magal have demonstrated.
As for why the .30 Carbine isn't offered in the AR platform, there have been several reasons already given. The Magal is a well designed weapon, and it still didn't catch on. I personally would like to see an AK version of the Bizon chambered in .30 Carbine, but it probably isn't going to happen either.