because there were very few automatic weapons sold before the sale of machine guns was restricted in 1934, and made practically illegal in 1968.
there are remarkably few grenade crimes in america too, since grenades were put on the nono list in 1934 as well. at the time, restriction of machineguns (expensive and rarely used weapons) and grenades (almost unheardof outside the military) was considered reasonable and proper for a civilized society, particularly in the heyday of the motorized bandit gangs like the dillinger gang and bonnie and clyde, but if you tried to pass that same law today you would fail. too many poeople have seen that reasonable measures are just the first move, and the goalposts shift with alarming regularity. the old playbook of "Think Of The Children!" and "only wingnuts and birchers would want those kinds of MURDERGUNS!" will fall flat. even with the Brahmin in Chief's personal auctoritas pushing the bill forward.
his gravitas doesnt move the people like it used to, since he has revealed himself to be a stuffed shirt.