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)
Well, because fiat currency has been around for centuries and has built up enough trust in that time to be used as a reasonably stable exchange of value. In many countries fiat currency might be the money used but a business can choose to accept whatever payment method they choose and obviously they go for the ones most convenient to the customer. A major obstacle for wider scale adoption on everyday transactions will be, in the current form of Bitcoin, the fact that you have to wait a variable amount of time for the blockchain to confirm enough for the business to be happy that you're not going to double spend and leave them with nothing.