They haven't tried because they know they can't and it would be deeply embarrassing to try and fail. If you understand how a decentralized technology works you know that it has no off switch, it's everywhere and nowhere, it's invisible and pervasive. Can governments stop drugs, bittorrent or ants? No they can't so they won't.

Bitcoin could have been stopped at the beginning, now the window opportunity has closed.

Besides, 3 out of 4 presidential candidates support Bitcoin and even Trump owns eth. It seems that those very governments are the stronger proponents of hard money now that the inevitable demise of the dollar seems near