China is a sovereign nation. For good historical reasons, it is sensitive to any suggestion that it isn't, or that other nations can ignore Chinese sovereignty. Read up on the Opium Wars, the unequal treaties, the Boxer Rebellion.
So it is one thing for Chinese officials to invite foreign researchers and to share data, and quite another thing to give in to demands by a geopolitical rival that the rival's researchers should be allowed to roam at will.
Especially if, as it appears, Chinese domestic politics slowed the effective response against the pandemic. With that I mean an unwillingness of local officials to report a possible problem to their superiors for fear of looking bad.
But that's easy to say in hindsight. I remember the reports in Western media which blasted the first Chinese quarantine measures as authoritarian, excessive, and even barbaric. Can you imagine what they would have said if Wuhan had been quarantined at the first hint of trouble?