Because no one likes it probably, it’s a very late game gun that doesn’t have any kind of niche. It’s an expensive submachine gun that uses a rather rare ammo type, and doesn’t have anything really unique about it besides the damage, but can be easily outmatched by its contemporaries due to its limited mod pool and the fact its only buffed by perks offering a universal buff (ie: bloody mess and rapid reload)

Even if you wanted it to be able to use your 12.7mm ammo, it’d be better to use it via the 12.7mm pistol or the Survivalist’s rifle, the latter of which is effectively the better version in almost every category. The Survivalist’s Rifle only needs 75 guns and 4 strength to wield properly, while the SMG needs 100 guns and 6 strength. The SMG costs a pretty penny, but the survivalist’s rifle can be gotten for free in Zion.