Remember, the (very flawed) reasoning behind the NFA was to circumvent the Constitution in order to ban the weapons that were commonly being used by organized crime. That meant suppressors, Tommy Guns, sawed off shotguns, and handguns. SBRs and sawed off shotties can pretty easily be carried under a trench coat if you're out on a hit. Handguns were very close to being NFA, but because were commonly carried for protection by law abiding citizens, Congress knew the NFA would have trouble standing against an inevitable challenge, so they backed off of that. However, as is often the view of ARs in contemporary America, Tommy Guns and SBRs were seen as only useful if you were soldier or a gangster.
Really stupid, and it sucks. I would love to have 12.5" barrel on my AR that didn't require an extra $200 and a form to the ATF every time I move or want to take it out of state for any reason.