Bitcoins will be hard for them to stop without shutting down the entire internet. Should a government act against bitcoin, it would drive the price through the roof.
There are already a few governments which have acted against bitcoin, and how do you think has that helped bitcoin? It is thrown around in the media negatively, residents of a country are thereby even restricted to use it because it is banned in their country now. Also, bitcoins doesn't have such a strong backing by a lot of people that the whole internet will be fighting for them. I don't think the internet or the people care about bitcoin as much as you believe they do.
That is the main reason that bitcoin is not into mainstream, as many governments have already banned the use of bitcoins in their countries, and the rest care is taken by the media who is continuously involved in spreading negative views related to bitcoins and that is the reason bitcoin is not gaining popularity as people have many other options in compare to bitcoins.