Why Bitcoin isn't adopted yet in most legimate websites?
consideration of course there must be from the buying and selling website, most websites don't want to take the risk to make transactions with Bitcoin, there are also those who really understand how Bitcoin works.
I think the biggest reason for not adopting Bitcoin by certain websites, is due to pressure from the government which is still confused about Bitcoin, maybe if the government declares something legal against Bitcoin, many websites adopt Bitcoin in their goods transactions.
This has a point since it isn't legalized. Imagine a website accepting bitcoin there and users in another country banning it. This will lead to their website having a problem in that country, so it is best to not adopt it as of the moment so that they still have clients. Also, fees right now are getting higher and higher, making it difficult to pay them if you buy a small amount. But basically, you can buy everything with bitcoin if you change it with fiat, but in terms of bitcoin online, there are a lot of websites now that accept bitcoin as payment.