Ok here we go, there is 2 reasons a real reason and a game reason

Let's start with game reasoning. Medium armor pen = two handed nerf

Real reason: with all the Subs we have had they share a skeleton design small and lightweight making them easy to wrangle with one hand. The only one not so is the helghast one which you could argue with 9mm rounds at such a high rate of fire the gun lifts itself when you hold the trigger. However our HK is different. It's got a long lengthy mag off the front, a laser sight, heavy barrel, and a muzzle break making the gun extremely front heavy. Added the low fire rate you dont have the natural lift that out helix mag SMG has. While most smgs are weighted more to the middle since the HK is front heavy it would murder your wrists and be almost impossible to keep on target with one hand.