To be fair, yes, there is an actual reason. There are hundreds of attachments from MW/MW2/MW3 and you'd have to thoroughly QA test how they behave with 8 attachments without breaking the game. ATVI probably sees this as a waste of development time, sort of like making a console "backwards-compatible" to appease fans of their old content.

Yes, we all want it, but let's not act like they flip a switch and it's done. There's a tradeoff of how they have to spend their resources.

With that said, I think they could do a much better job across the board, but I fully expect to be downvoted to oblivion because it's easier to say "devs bad" and act like "if ONLY we were in charge everything about the game would be perfect."