Perhaps because the Chinese company "Fosun Pharmaceutical", as the other sponsor[1] of this vaccine, is the one which has the right to distribute it in China?

[1]: https://en.wikipedia.org/wiki/BNT162b2#Funding

Pfizer will start another smaller trial in China to generate local safety data, which will be combined with its global data as it seeks regulatory approval in china. [1]

[1]: https://www.scmp.com/news/china/science/article/3110042/coro...

China has its' own regulatory requirements which typically (I don't know in this case if China is asking for it) requires a specific trial in an Asian population in order to get approval.

I'm assuming that China is probably not waiving that requirement since they have their own vaccines.

China is making their own vaccines.

Because China already has a "national" vaccine, and they see no chance getting through Chinese bureaucracy for such a politicised deal?