Why Bitcoin isn't adopted yet in most legimate websites?
Because its usage is not mandatory while fiat currency's is.
As for what you can buy with Bitcoin:
Lots of stuff. C'mon man, we're not in 2010-11 when no one knew it. Thousands of businesses accept it, a country made it legal tender. Even on places where it isn't accepted, you can purchase using Bitcoin: You just need to find someone who'll accept purchasing it for you in exchange for bitcoins. (e.g., bitrefill)
I hope Bitcoin is a digital currency which is also known as soft-currency that can be used to transact all goods and services internationally.
And also there are some countries that uses physical Bitcoin to make transaction for goods and services just like Fiat currencies.
That is to say Bitcoin currency has no limitations in the buying and selling enterprises.