So why are digital threats not allowed on rifles and lmgs?

I feel like all the best weapons have access to this attachment, I don't this attachment is making them the best weapons but I do question the balance reasoning, if everyone's gonna use an r-99 why also give it the digital threat and leave something like the r-301 to feel like a side grade? Someone gonna say those weapons are for longer ranges but past a certain distance you either want a sniper or you just avoid a fight till you can push in (most people will probably just push in snipers are dead) Most people keep a shotgun which all can use the attachment, most people end the game with a sub machine gun which all can use the attachment so I just wanna know the balance reason to limit the use of that attachment, I mean it's also a one time sight.