Why would a government impose as legal tender a currency that it can not print and therefore can not profit from it?
As others have pointed out, it happens in certain situations. Even worse, governmnets are willing rely on currencies issued by other governments in those cases: Not only do they not hold the pwoer over the currency as with bitcoin, there's even someone else holding that power.
Here's a few examples:
Montenegro and Kosovo used the German Mark and now use the Euro.
Many small states use the Euro.
Lichtenstein uses the Swiss Franc.
East Timor and Ecuador use the USD.
With the exception of really tiny states this usually happens in times of turmoil, civil war and major economic crisis. We could use that knowledge to make Bitcoin more widespread and to create a suitable global environment for further acceptance of Bitcoin.
Bitcoin is the first non-government money system that I think would be suitable for becoming an official currency. The main problem right now is that the market is so tiny. No one can really throw the Euro or the Dollar around, but with a couple million dollars someone could easily take BTC on a violent rollercoaster ride until everyone runs away puking.
These little countries use major reserve currencies because they are too big to mess with, not because they really care about the currency itself, or the nation that issues it. Oh, and don't forget geography. Using the same money as your neighbors is another big advantage.
So, to answer the OP, bitcoin needs to grow first. And then it will be suitable to be granted legal tender status by some nation.