Maybe if Amazon accepts bitcoin then there will be some big companies like visa and mastercard will be harmed because there are no more consumers who shop using the services they provide, besides if Amazon accepts bitcoin then they will also not be used by people living in a country which prohibits the use of bitcoin, obviously it will not be so profitable they will choose to use conventional payment methods using a credit card.
Well, they might have other reasons. We know that there are some transaction fees and in terms of an online shop, you must be used by that. Imagine if you are owning an online shop and a lot of people ordered but then they want a refund, what you will do is to return the payment they paid you, though as I said, the fees will be a problem. You did not just lose via refund you also lose a fee, it will be better if that is just few, but they are not.
Amazon has to pay salaries for its employees. If Bitcoin falls critically, how will they do it? It will affect the efficiency of the company work and the wages of people negatively. Amazon is not going to take risks.