The lab leak theory is popular because of politics and ignoring the down sides of potential error. The lab leak theory posits that natural disasters such as have happened throughout history no longer happen because all events are shaped by the hands of man. The lab leak theory is based on the idea that establishing guilt brings justice. The lab leak theory is based on a generalized loathing of China. The lab leak theory ignores the history of ongoing transfers of animal viruses to man in favor of the view that it is different this time. Garbage in results in garbage out and the lab leak theory assumes that an incorrect idea will result in correct political action.
Perhaps the greatest tragedy here is that people are appealing to ideas about justice by saying this was a forbidden struggle that is a big fight, yet ignoring the most important realities of justice. If you really want justice then you need a coherent statement of the offense, there should be a fair hearing with representatives of all sides, there should be impartial review whether that be trained judges or a selected jury of peers or whatever else, and so on. We know what justice looks like and any serious introspection will show that this shrill advocacy of the lab leak theory is just more social media garbage like q anon and the rest. If you want justice then you will have to submit to the kind of impartiality that brings justice, but that isn't what we are talking about here.
It was ridiculed because a Republican Senator popularized it, not because of evidence.
The way you know it had nothing to do with the evidence or lack thereof is that people who respond based on evidence don't usually respond with ridicule, and people who respond based on tribal affiliation usually do.
There were definitely some people saying "this is possible, but on balance unlikely given what we know today", but for the most part words like "crackpot" and "incompetent" and even "racist" were used instead. That's not what arguing from a place of facts sounds like.