Because the similar way of describing the US system is fundamentally untrue. There is no part of the US Constitution that specifies, by name, which party shall rule.
Could that change? Sure. Especially with so many people apathetic to what happens in China.
Did I say that the US has a healthy and democratic political life?
Of course the US has its problems. Just look at the government now.
It's just not a constitutionally-defined single-party totalitarian state.