Dexron-IV was a spec that was never released. Dexron-V would get folks confused with Mercon-V (Shucks, Gomer, all that V stuff is the same, ain't it?). Thus, Dexron-VI.
Dexron licenses from GM expire each year. There isn't much demand for ATF for the newest cars except maybe for warranty repairs at this point. GM recommends 100,000 mile ATF changes, and there might be some aftermarket demand for Dexron-VI when the transmissions using Dex-VI start reaching that mileage.
Ken