A lot of websites, services, and stores online are accepting Bitcoin for payment, even Microsoft has started accepting Bitcoin as payment for digital purchases on their store and Xbox store and it is one of the biggest companies of the world. So I'm pretty sure that major eCommerce platforms such as Amazon will start accepting it as well in the future.

The biggest reason that I can think of about why they are not using it right now is the regulatory complications and maybe the excessive taxation from the governments for Bitcoin in developed countries such as the USA where most of these companies operate, and they wouldn't want to pay a lot of their revenue in taxes only for accepting Bitcoin.