Simply, because it's not mandatory while fiat currency's usage 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)