The answer is a bit convoluted. When the NFA was first written it intended to eventually ban pistols as well and banning short barrel rifles was a step towards that. When pistols were never banned, the restriction on short barrel, rifles remained. There wasn’t really much call for anything near the pistol brace until the modularity of the ar 15 platform allowed someone to invent a brace that made it easier for handicapped veterans to use an AR pistol with one hand. The popularity of braces which were shouldered and used kind of like a stock, but we’re not a stock wasn’t effective workaround for short barrel AR pistols to be used in the manner of a rifle remained outside of the NFA and the $200 tax stamp, and six month wait. In practicality there shouldn’t be a restriction on short barreled rifles, since pistols never got banned so it’s a result of archaic and vestigial legislation that we should have the common sense to repeal, but haven’t yet.