There's a lot of reasons why people prefer to use the btc over fiat.
I think one of their reason why they stick to btc, its because btc has a lot of great features than the fiat and its also much more profitable than the fiat.
Profitable? there is high risk of losing money too
I think most user prefer bitcoin for because it cost less to send money and you don't have to pay taxes for transaction made using bitcoin.
if you have the wealth and income in the real world. of course everything you earn and your riches such as cars and motorcycles will be taxed. whereas regardless of the wealth in the bitcoin will not be taxed