That's because the US enacted a law called the Wolf Amendment in 2011, which prevents NASA from using government funds to cooperate directly with China.
unless it has certification from the FBI that such collaboration poses no threats to national security or risks inadvertently leaking space-related tech or data.
They're almost certainly never going to get authorized, the whole point of researching moon rocks is to learn space data, and of China requires that you share the data you learn from the rocks you're studying, it's impossible