What is Stopping?
No one or nothing stopping them.
Why government Don't kill btc?
Because they can't not unless the whole world will stand as one and decide to ban the usage of Bitcoin and close all exchanges in the world,then maybe little by little this will die,but very impossible to happen because never that will happen about the world will decide as one specially in something that others believe that it is Helpful like Bitcoin.
Why They protecting the btc?
Nope no government is broadcasting their full support for bitcoin though there are some country that let this operate in their areas.
Its Becouse They own Secretly btc?
Like They did with Gold in USA They banned Gold then They Secretly bought Gold in history
Either Government is owning Bitcoin secretly or not,it doesnt matter because still World will decide about what crypto future is.