A couple reasons, from the perspective of an infectious disease epidemiologist who isn't particularly a fan of Gain of Function studies:
1) "The lab leak theory" is actually a collection of theories, from the fairly reasonable possibility of an accidental release as part of a lab accident down to a engineered Chinese bioweapon. What any individual means by "lab leak" tends to be whatever is serving their political purposes as the time, and they borrow evidence from each other in a way that's really flawed, etc.
2) There's a lot of personal attacks built into the evidence - whether or not Ralph Baric is a likable guy, for example, fed into a lot of whether or not his lab was "involved".
3) A lot of the advocates for the lab leak hypothesis don't come from epidemiology/public health/virology, but from the intelligence community, the tech world, etc. who tended to reach for "Oh, it's obviously this..." and who reacted poorly to the idea that it might not be that simple and straightforward. Both sides of that exchange burned a lot of good will with each other early on enough that the engagement has, I think, spiraled somewhere not particularly productive.
4) It got coopted by crackpots and conspiracy theorists, as well as those interested in abdicating their own role in the pandemic.
5) The biggest reason, in my mind, is that it is a "fast" solution that gets a lot of press, and for which the alternative explanation - the one that has been true for the last two coronavirus outbreaks - is something that will take the better part of decades to resolve, if it ever does. It's also by and large not relevant for the current state of the pandemic. That creates the sense that there's a whole mountain of evidence behind it, when in reality it's a theory that is going to be rather difficult and expensive to falsify, one way or the other.
1) "The lab leak theory" is actually a collection of theories, from the fairly reasonable possibility of an accidental release as part of a lab accident down to a engineered Chinese bioweapon. What any individual means by "lab leak" tends to be whatever is serving their political purposes as the time, and they borrow evidence from each other in a way that's really flawed, etc.
2) There's a lot of personal attacks built into the evidence - whether or not Ralph Baric is a likable guy, for example, fed into a lot of whether or not his lab was "involved".
3) A lot of the advocates for the lab leak hypothesis don't come from epidemiology/public health/virology, but from the intelligence community, the tech world, etc. who tended to reach for "Oh, it's obviously this..." and who reacted poorly to the idea that it might not be that simple and straightforward. Both sides of that exchange burned a lot of good will with each other early on enough that the engagement has, I think, spiraled somewhere not particularly productive.
4) It got coopted by crackpots and conspiracy theorists, as well as those interested in abdicating their own role in the pandemic.
5) The biggest reason, in my mind, is that it is a "fast" solution that gets a lot of press, and for which the alternative explanation - the one that has been true for the last two coronavirus outbreaks - is something that will take the better part of decades to resolve, if it ever does. It's also by and large not relevant for the current state of the pandemic. That creates the sense that there's a whole mountain of evidence behind it, when in reality it's a theory that is going to be rather difficult and expensive to falsify, one way or the other.