Nevertheless, the gain of function research with coronaviruses has been documented in the peer-reviewed literature.
The lack of new information from the Wuhan Institute, despite its longstanding research activities, is probably the most compelling evidence in support of the lab escape hypothesis.
If the evidence pointed elsewhere, it would be released. The most likely explanation is that the Institute's fingerprints are all over this thing.
The second most compelling evidence is that to date the reservoir species has not been found.
As an uninformed reader, some possibilities I can come up with are the following:
1. Despite zoonotic origin, China feels its stature in the world is much greater than in 2002-2003, so it no longer feels obliged to allow international investigation into origins of the pandemic in spite of political pressure. For example, Wikipedia says: "After intense pressure, Chinese officials allowed international officials to investigate the situation there."
2. Despite zoonotic origin, COVID-19 has much higher death toll so CPC wants to save face by either hiding evidence or preventing investigations into a potential reservoir species. OTOH, SARS had lower death toll, so they didn't have reason to hide anything.
3. Despite zoonotic origin, Xi Jinping's regime is less transparent than that of Hu Jintao which was in power for SARS, so this time they want to either hide evidence or prevent investigations into a potential reservoir species.
4. Despite zoonotic origin and best efforts of Chinese scientists, a reservoir species has not been identified yet. Is it possible that it may take longer this time around?
5. Lab leak hypothesis: there is no natural, reservoir species to be found.
Personally, I think Reasons 1, 2 and 3 are strongest, because CPC feels like it has to play to its domestic audience and letting international investigators in may feel like a loss of face. However, this doesn't add up, because they already let WHO investigator in, albeit with very stringent limits and not letting them access raw data. I'd be much obliged if a learned person in this topic could tell me what kind of prior they'd put on each of these possible explanations.
[1] First human case of SARS was in Nov 2002, and scientists were able to isolate SARS in civets in late May 2003, so a reservoir species was identified in 6 months. It took 10 months to identify that for MERS (Sept 2012 -> August 2013).