Nothing. Anyone could start their own bitcoin2. But new instances of a digital currency cannot debase other instances. So that isn't a viable way for a government to capture bitcoin, or to debase the value of the original bitcoin.
In fact, were I running a banana republic with a worthless currency, I'd be tempted to try using a new "bitcoin-space" as a currency with strong protections against being debased. But that has plusses and minuses if you are a government.
The reasons people are using this particular instance of bitcoin is that it was launched successfully enough for people to have sufficient confidence. The mining mechanism is one key element.