Well, in their view it is not a big enough threat to attack and spend millions on gaining a majority of the hashing power. Once it is determined to be a threat it will be too large for them to buy their way in to control. Plus the second they gain control is when all of the money floods out of Bitcoin and in to a different crypto. That is the beauty of it, we are not forced in to using the currency so attacking it results in wasted money as the users move out of Bitcoin and in to a different crypto-currency.
The agencies would have an interest to make bitcoin big. They can use it for their own operations if needed and they can follow the trail of the money. Since everyone know that, for example the nsa, controls the internet traffic of whole countries. So it would be not really hard to fight terrorism and similar things like that.
Well, pushing in bad code that is not found might be another nice gimmick but so far I think it was not successfull. Too many observe things closely.