Please explain the widespread use throughout military, law enforcement then?
Many varmit / small game hunters prefer them primarily to have a smaller footprint in their backpack, lighter weight, less chance catching on branches and such in the woods.
I'm not in the military (any more) nor in law enforcement. I don't regularly ride in a Humvee with a rifle and need to get in or out in a hurry & possibly under fire. I don't regularly kick down doors and clear buildings for a living either. All applications where I can see the benefit of an SBR, that don't apply to me and pretty much everyone else that's not military or LE.
I don't hunt either, so I can't really speak to that. But I'd think varmint hunters wouldn't want to give up the velocity and range you sacrifice with an SBR, seeing as most factory 'varmint' style ARs have 18"+ bbls on them. But again, I'm not a varmint hunter so that's just my opinion.
So for me, it might be a cool 'toy' but that's about it. Also let's not forget the ballistics of a .223 round and how that works, and velocity is absolutely the .223's best friend, and something you lose in an SBR. I'd have to dig up the info again, but there is a velocity threshold where a .223 round's effectiveness starts to be dramatically reduced.
Personally I think an SMG is better suited to that kind of task, and if I could I'd have something like a Kriss Vector instead of chopped AR. But again, that's just a layman's opinion. YMMV