Wade's article explains that. With SARS-1 they did find the animal transmission path very quickly (within months). Part of the argument here is that despite an overwhelmingly large and well incentivised search, no such path has been found for SARS-2. Meanwhile there is microbiological evidence that is suggestive of lab work.

Unfortunately due to China's stonewalling there is no way to prove anything beyond doubt. However, the balance of probabilities at this point says that the WIV was doing GOF research on these sorts of viruses and one escaped. One reason to believe this is likely is that they apparently were not working at especially high biosafety levels. Just ordinary lab coats and gloves.