They probably believe that it'll make the loadout overpowered, just like you can't use FMJ with the MMS
But if you could, people would cry "OP" the second they die to it. I'm not exactly disagreeing with you, I think you should be able to use them together if you want, but everything is OP in COD anyway so I guess they were just trying to limit the inevitable flow of messageboard tears a little bit wherever they could.
The hybrid scope was probably originally mapped to the left d-pad like in MW3 so that's why it can't be combined with select fire or the grenade launcher. I guess Treyarch forgot to update it's compatibility when they moved the zoom toggle to the thumbstick. That's my theory anyway.
People don't use it because it's the last attachment. And when you unlock it it's time to prestige the gun. By the time you have done all the camos most guns usually haven't been prestiged to max yet. And when it's done it's time to move on to another gun.
At least that's why I don't use it. I will in due time of course, but only because of the calling card. I find the optic attachments to be a waste.
Now you're blushing and you have an erection, no one has enough blood.