Why can't they use super computers to mine all the bitcoins?
A supercomputer is way slower than mining with ASICs. A supercomputer only has much CPU power, not even GPU power and ASICs are way more powerfull than GPUs. ASICs represent the hashing algorithm as hardware which means they can't do anything else, that's why they are so fast.

At http://bitcoinwatch.com/ you can see the current network hashrate in PetaFLOPS, which is 1534782.24 at the moment. Currently the fastes Supercomputer Tianhe-2 has 33.86 petaFLOPS.


  but for a govt agency all that money comes from taxes so ...


If they would use all the money to buy ASICs and start mining bitcoins, it still would not be enough because of the difficulty factor. But they would be able to get a lot of blocks because the difficulty only changes every 2016 blocks.