Understand that the NFA was written in the '30s when FDR and his social engineers were extremely active. They even wanted to remove our Bill of Rights, and replace it with a new one which was very communistic, right to employment, etc...
As such when the NFA was being developed, it was extremely restrictive. It even included handguns, but they were removed as part of a deal with the Texas delegation (one thing Texans can REALLY be proud of). Also, there were very few semi autos as we know them now. You can be DAMN SURE that if weapons like our semis were available then, they'd be NFA too...
From a gun prohibitionist point of view, consider that handguns were originally written into the NFA when wondering about the measurement logic of SBRs, SBSs or AOWs. They make no sense when hanguns are legal, but that wasn't what they originally wanted.