A government has many tools to attack bitcoin if they want. A 51% attack isn't one of those 
I am well aware that it is not the first recourse. But most of those other methods will not completely eliminate it, especially if facing hyper inflation. It would be illegal but it would thrive just like alcohol or drug under various prohibitions.
If such a situation occurs where the currency has already been deemed illegal and is purely an underground thing in a "communist utopia" the a 51% attack might be resorted to simply because there is no defense against it.
I think the only reason no one has done that is because there are laws against it. If for example it was perfectly legal to do that, then it would have already been done numerous times.
There aren't any laws protecting Bitcoin however and there's a large number of tech savvy users.
There aren't any laws protecting Bitcoin however and there's a large number of tech savvy users.
a 51% Attack was successfully executed against feathercoin.