Olympic Arms and the OA-93 are to blame for that. A couple hundred AK pistols existed around the same time but ATF didn't make the ruling until seeing the OA-93 in 7.62x39 at SHOT in '94.We're fortunate that the ATF designated 7.62x39 as "armour piercing handgun ammo" after folks started to build AK pistols.
If not, we wouldn't see the ammo that we get as it would go to the US.