I never understood why things like SBR's and suppressors were NFA. Oh no, my rifle is 2 inches shorter, it's now below the magic line and is capable of nothing but evil. Heaven forbid if I want to save some of my hearing by using a suppressor.
Because they were seen as "gangsters' weapons" at the time; it's no different than the push to reinstate the AWB today.
Originally, all handguns were going to be covered by the NFA as well, but there wasn't enough legislative support for that. Count your blessings.