The current "fad" in shooting AR's is to have the support hand almost fully extended and the thumb gripping over the top of the tube.
On SBR's that means using a long tube, sometimes right up to or over the flashhider. Linear comps allow that without the blast (which is considerable on an SBR) being directed into the support hand.
The blast being directed downrange also makes the gun more comfortable to shoot, hearing wise. Even with earpro these little guys are LOUD. Add a muzzle brake and they are REALLY LOUD. With a linear comp, not as bad.
Not "Boutique"...just part of the evolution in the way things are modified to improve performance.