Universal doesn't completely control Universal Beijing (just like Disney and DL Shanghai); it's a complicated structure with dual ownership levels (operating / asset ownership - majority of assets owned by Chinese partner), and I'd assume that there's all sorts of content restrictions in the contracts that give the Chinese partial owner an important say in the content in the park.
The Chinese leader involved in the project mentioned that around 30-35% of the content would be Chinese/local and that they might have to build to that over time starting from a lower base. Either way, content in China is a completely different and more complicated situation than elsewhere like Japan/US where Universal fully controls the parks.
My impression is that Nintendo in Beijing will have to be negotiated after the first park is fully built. Universal+Nintendo will probably gauge how successful it is in the US and then show the Chinese partner that it's a good idea to bring it to Beijing.
But these things can be impacted by politics as well (there's all sorts of situations where Japanese/Korean content is restricted in China when geopolitical tensions rise), that's probably why Nintendo and Universal are waiting and seeing on future Nintendo Worlds outside of Japan/US.
