Because the money levels being talked about are large enough that you could trivially destroy it. Which means your entire foreign exchange rate is subject to whether someone wants to screw with the blockchain.

Which means at any junction, some third actor can hold the entire currency hostage - unless the governments invest in huge amounts of hashing hardware to protect it. Which is (1) a colossal waste of money and energy and (2) would, in turn, give them the capability to do the exact same thing.

Bitcoin will never be a currency held or traded by governments.