Could be budgetary reasons. Panavision rentals are really expensive, but then again this is Star Wars so I doubt they needed to save money.
Do you know how long it took to shoot ROTJ? If the shoot was longer than ANH and ESB they might have reasoned that it was cheaper in the long run to buy some Arriflex cameras rather than rent Panavision equipment for the duration of the shoot.

Or it might have to do with the director, maybe Marquand preferred Arriflex? He did a lot of television before SW, most of which was probably shoot with Arriflex cameras. So maybe he was more used to their equipment. Then again, I saw on IMDb that his previous film before ROTJ, Eye of the Needle, was shot with Panavision cameras, so I really don’t know.

EDIT:
I don’t think it would really matter if they filmed much of it in England. ANH and ESB were also mostly shot in London. But then again, they may have just seen an opportunity to save some money. Whatever the reason was, it probably made sense to the producers back then.