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.
well people are mostly using bitcoin as an investment and so far it has been a great investment for those who are trading it day to day and also for those that have been going long by just buying some and putting it away in a cold storage for ages.
but people are not just preferring bitcoin because it is an investment they do it because bitcoin is decentralized and also it gives them more anonymity than fiat can give them. also merchants prefer it because it has no charge-back.