The email I just received from Amazon states Visa's fees as the reason. I don't know what the relative !market shares are, but Visa credit cards are incredibly common here. I'd say more than half the credit cards I've ever had have been visa.
etc. you get the idea, so if VISA credit has low transaction count, then expenses for the transactions are a lot higher than for VISA debit transactions - if the user base for VISA credit is couple of tiers lower than for rest of cards, it makes sense for these "rare" users to switch to other cards, so Amazaon does not have to subsidize their loss of income from VISA credit cards.