A lot of e-commerce sites have already started to accept Crypto (BTC) as a payment method along with others. I think you did not check before asking this question. Well, the most famous E-commerce stores like eBay or Amazon have not yet accepted BTC as a payment methods, and its very logical as they don't have to, as most of there customers are already happy with the debit or credit card system. They don't need crypto.

And TBH I don't think many people would be shopping things online via crypto if these sites would have allowed them to. BTW here is the list of 23 online shopping sites where you can use BTC for payment methods (Sites list). If you want to open a store on Shopify or your own site then you can also integrate the BTC payment method in it easily with not so much difficulties.