Best reason: Because Vision is OP as hell and if he were not nerfed at the outset, how do you get the Mind Stone from him? So in short; because the writers said so. Vision's character suffers from Superman-writeritis where it's difficult to write around a hero that is damn near God-like.
However, Marvel has so many Superman characters that I'd argue that it would be rather easy to write something around Vision that shows his power, instead of having him incubate the Mind Stone until Thanos shows up to take it. I mean why not have an incredible fight with the Black Order until Glave lands a lucky strike on him? It makes more sense than a giant ship silently creeping up on them and then Glave sneak attacking an ever-aware God-droid.
But again, the issue is that Vision/God-Droid is a bit too OP for the Black Order without injury. In combination with Wanda, that fight would have been hella one-sided. Combine that with the fact that the Time Stone would also be impossible to get because the combination of Wanda, Vision and Strange is enough to prevent anyone from getting those two stones. Then bring in Thor once he gets God-level power and now the movie is pretty much over with Thanos only getting four stones.
Even Strange was nerfed at the outset, but I'd argue that he didn't even realize how OP he could be until he ran into a superior sorcerer in Ebony Maw. I think that is when he realized his full capabilities after seeing another being with like-powers be so adept. Its like when you play a sport and see someone with like-skill do something you could've always done, but you never did it only because you hadn't seen an example (i.e. crossover dribble into a fadeaway shot).
And of course Thor couldn't be OP at the outset or Thanos's quest would have ended during the opening credits. So again, Vision was nerfed because of writing and nothing more. All the talk of what the Glave can do according to the comics and stuff is all great information, but it means nothing in the context of the film, especially when you consider that he was blindsided in the most contrived manner possible. He was simply nerfed so the movie could work.