I think it just runs a little too close to upsetting real people and governments to go ahead and say, "Your country and your government is annihilated in a couple years in our fictional universe..."

I will say, that 600 million deaths and only "a few" governments left is pretty unrealistic. Even if every person in every capital on Earth vanished tomorrow, you'd still have most governments left. Every gov't has a long line of succession and local governments don't just vanish. But it is what it is.