Because China Gov owns around 200,000 btc and this is a lot. More than US government. By reasoning, their voice is bigger compared to US government in cryptocurrency.
Just like Bulgaria, IIRC, they've seized bitcoins I can't remember if it was the same amount as China's.
Should we take china words seriously? No. Because they been trying to ban since year 2013. Then they un-ban and then they ban again. History repeats.
Not really.
They're doing that as if people who are aware of their move would be scared and try to dump those bitcoins they own. I think that they'll be holders and wouldn't stop to manipulate bitcoin.