1) they don't see it as a threat (ex: Powell comparing it to gold rather than USD)
2) it's actually generating capital gains tax, jobs and revenue for the country (by creating business around it, etc)
3) US has auctioned off seized Bitcoin in the past, so it would be weird if they make something illegal after selling it themselves
Last, but not least, they can't ban it, they can ban the on/off ramps and cause short term disruption, but that wouldn't be sustainable in the long term, how do you stop millions of people from running some code on their computer (case in point, torrent is still very much alive)?
Also, if the US bans it then it's still valuable in much of the rest of the world. Bitcoin is antifragile in that regard.
India, though, has floated such a ban.
https://www.reuters.com/article/uk-india-cryptocurrency-ban/...
And I don’t think it would be crazy to imagine China doing such a thing.