Because lots of bitcoin transactions are done in China. China plays an important role in cryptocurrencies. Any news from China about cryptocurrencies can affect the prices.
I'd hazard to guess that China accounts for a tiny fraction of Bitcoin transactions compared to places like the US. The only reason for Bitcoin to move there is to exchanges or miners moving rewards. There's no Bitcoin commerce, no gambling, not much of anything.
Its markets have been extremely important in the past, far more important than they proved to be when they were shut down. It was a mirage. And it is of course the capital of mining. There are some very deep pockets there.
Chinese agitation is the prime ammunition of market manipulators. It'll be interesting to see what they come up with next when China has faded away and the upcoming fork fails.