So while backing a currency with bitcoin (which is pointless in itself as you may as well just use bitcoin) might seem like a good idea it will never happen.
There would be plenty of reasons why a country would want to have it's currency backed by bitcoin
1 - Bitcoin is highly technical and a country's population may not understand how to properly keep their wallet safe. By having a currency that can be held people can understand how to protect it in the same way that they understand how to protect cash
2 - If a country does not have the infrastructure in place to support that level of connectedness to the internet. If a country used bitcoin as a currency then anyone wishing to spend their money must have access to the internet any where that a transaction is to take place. Even in modernized countries there are places without internet access that have transactions take place.
3 - Countries would need to rely on ISPs much too heavily for their economy. If access to the internet is disrupted then their country's economy would come to a standstill as no transactions could take place.
4 - Per capita income/assets may not be high enough for every citizen to be able to afford a smartphone that would be required to spend money with bitcoin.