this isn't the reason, they are not adopting it because bitcoin isn't a valide alternative for them, it does not bring any advantages over fiat, and they do not care about decentralization
and all this because they are ignorant and they do not want to learn about bitcoin, maybe they could find that they could save time and money if they use the blockchain instead of a standard fiat circuit
Corporations such as Ebay and Amazon have a number of deals with payment processors such as Visa / Master / Paypal, giving them an unfair advantage over the smaller players such as Overstock and Alibaba. If they adopt Bitcoin, then there will be no advantage for them, over the other players. So I can understand why they are reluctant on BTC.
BTC is still relatively new currency. People are just become aware of it.Consumer don't take risk of keping their money on bitcoin ,as it has a high risk of loss and they may be can't afford for their loss (if happened asit depend on private key) .