Government would want to get its share of the transactions through taxes or fees and due to the limitations of the bitcoin not being used all the way to purchase goods as there are only selected merchants that accepts it, people have no choice but to convert it to fiat and through this way, government would be able to trace the people who would convert it because more often than not, the platform that would be use for conversion is registered and licensed by the government so they need KYC.
I don't mind anyway paying taxes because it is my responsibility as a citizen but there are times when the taxes are just too much and then it would just go to the pockets of certain individuals because of corruption. It would be better if the system of paying taxes is in blockchain too and the flow of money would be traceable so that we can caught those corrupt people.