Why more Bitcoin users prefer bitcoin over fiat
means first reason is yes Bitcoin is tax less and to use bitcoin there is no restriction.
And now we got a lot of options to make money from Bitcoin.
And the price is also helping people to use more bitcoin over fiat.
And one more thing is in some country the currency value is very less for them this bitcoin will giving nice profit.
So people started to use Bitcoin than fiat currency.
Some people are criminals and they are doing money laundering. With bitcoin, they don't be followed with their financial transactions. This case let them prefer bitcoin over fiat money.