Why can't nations simply adopt bitcoin as the official legal tender??

For a very long time, we have always wondered why nations are afraid of making bitcoin a legal tender. Yes, we know they have central banks and we also know they want control. But wouldn't it be possible that nations use their reserves to purchase enough bitcoin to power their nations economies?

Alternatively, we have heard of some nations that are working to launch their own kind of crypto. Others like China seem to be working on their own blockchain.

But it still confuses me why nations/governments always seem to move slow on things that are as plain clear as cryptocurrency and bitcoin. Why??