The metaphysical throw some people have given is that the TV Watchers, Viewers of the time Twin Peaks was on did it, and this fact is in some ways part of the "weird" narrative in season 2, Fire Walk and season 3. The fact is that the viewers at the time hated the elongated nature of the mystery, while that and whatever was going on with the characters of twin peaks around the mystery were the point. Catching Laura Palmer's killer was not the point of the show as much as the search and other mysteries. So when the TV station/Studio got enough complaint letters that paraphrased to "GET ON WITH IT" they forced Lynch and Frost to come up with a who and break the mystery prematurely.
During my first watch I was able to guess the killer and the why, but I think I just got lucky. who did you think the killer was?
At this point, Lynch might have already dipped out fully IIRC, and it was mostly Frost's fort, but I would like to think they purposefully chose "The most obvious" character, the one people already pointed fingers likely in their letters to the station, just if nothing else, themselves giving the finger while the people they are doing it to are very pleased of the nice surprised of them being right. Given what Lynch did with Season 3 it would not be weirdest thing, and this mandate that was born of the TV viewers of the show absolutely destroyed the show. It had lost it's purpose and thus Lynch dipped out of the show where Frost more than less alone had to come up with whatever narrative threads and revive the mystery by building out branches of additional mystery behind the mystery but a lot of viewers also dipped out because they got what they wanted and now it sucked. Lynch Returns to make the finishing and best touches to season 2, then makes Fire Walk to be a weirder experience but in some deep analytical level also explained how Twin Peaks as a TV series (should have) worked. Then Season 3 happened and can be viewed in meta narrative level as a series existing because of later fans, already destroyed by first fans, being forced into existence that makes little sense.
My vote, given it is something absolutely stated by Lynch few times, is the TV Viewers. Unlike
@ffviiremake sees it, the viewers at the time did not appreciate some of the convention breaking the show was doing, and some complained loudly enough so their complaints broke the show. It would be only years later when shows inspired by Twin Peaks in part would be accepted more widely despite breaking the conventions of one episode mystery and other conventions Twin Peaks was breaking. Lost, perhaps, a big first example I can think of. Meanwhile, even during this many shows kept to a TV Structure for TV audience and being minimal in things that would later become Prison Break, Breaking Bad etc.
