That is a very good question. Frank C. Barnes deals with it obliquely in Cartridges of the World. He doesn't state outright, but it looks like the advantages of aluminum are that it is much lighter and cheaper than brass, while steel is just cheaper. (Both have the disadvantage of being difficult or impossible to reload.) He dismisses the notion that case strength is an important factor, noting that aluminum is in use in 20mm and 30mm autocannon ammunition - when your weapon burns 3000 or 6000 rounds per minute, weight is at a serious premium, and when your cartridges are that large, material cost is as well.
I suspect that the difference for us Kel-Tec fanciers has to do with chamber pressure, and how the cartridge material behaves under heat and stress. Aluminum cases seem to work fine in most semi-automatic pistols; with the 9mm Luger and .40 S&W stuff we're talking about, those are all locked-breech weapons with very few exceptions. The big SUB 2000 can be built with an unlocked breech (straight blowback, similar to a lot of .32-caliber pistols or 9mm submachineguns), which apparently works fine for ordinary (expensive, reloadable) brass cartridges but not so much for aluminum.
"Steel and aluminum cases do not resize and reform as completely as brass" says Barnes.
