With a modern fiat financial system, the state can charge taxes automatically. Its hard for me to imagine such a system based on Bitcoin that will allow to do this. In addition, it needs to be built, and it will take more than 5 years.
And that is the issue governments want to know exactly how much money you are making so they can charge you all the taxes that they can, that is why even if they are the ones that print the cash that you use they want to limit how much cash you can use in each transaction and if possible they want to eliminate cash completely.
But now bitcoin exists and it is clear that it opposes their wishes and while they would like to destroy it they do not really know how to do that.