I suspect the answer here is politics. Eearly into the pandemic, the lab-leak theory was being conveyed mostly by conservative voices.

This time article dives into this idea: https://time.com/6051414/donald-trump-wuhan-laboratory-leak/

20 million deaths, according to excess deaths counts, because of scientists doing experiments they knew were dangerous?

It would destroy confidence in virology, they would be regarded as mass murderers.

Diplomatically, it would have required more of a confrontation with China, which would could lead to less trade and economic issues. Had it been a lab leak in a much smaller country? I could see both parties taking a much more confrontational role towards that country.