There is no normal procedure for the outbreak of a pandemic like Covid-19. Everything was kind of going crazy.

If you want to take the more charitable view, you could say the Chinese government was desperate to stop the virus from spreading and put a much higher priority on wiping out potential sources of infection (eg, the animals) and preventing spread (by locking down) than in getting researchers moving around to sample those animals before they were killed.

If you were going to be less charitable, you might say the Chinese government maybe didn't want conclusive proof to be found that it's mess of wet markets had, predictably, caused another zoonotic disease outbreak and was quite happy to see all the potential evidence killed off before it could be sampled....especially by any independent outside researchers.

One thing that's worth noting is that while it's academically interesting to know the exact path and intermediate hosts that covid took to get into people, it's actually not that important from a public health perspective. We know the broad strokes pretty well even without having all the intermediate steps perfectly filled in.