I suppose one could come up with a lot of theories as to why.
They're jealous: Men who are unsuccessful with women but refuse to try PUA methods because they believe they are misogynistic need to feel better about why they are unsuccessful with women. So the logic becomes "oh sure I could do that stuff, but I'm way too respectful/feminist/against the patriarchy to do that".
They put women on a pedestal: A lot of men (and I've met plenty of them) seem to have this weird perception that women NEVER make bad dating decisions, or at the very least never make dating decisions that might not be conducive to what they claim they are looking for. For them, a woman who sleeps with a guy because he dresses in ridiculous clothing and uses a bunch of techniques he learned at a seminar undermines that idea. It REALLY undermines it further because so many of the PUA techniques are based upon the complete opposite of putting women on a pedestal, so this damages their worldview.
They believe BS about PUAs: I have no doubt there are certain PUAs who advocate extraordinarily questionable methods, but some of the things I've heard about PUAs (especially on websites like Jezebel) are just so far-fetched they'd be hilarious if people didn't believe them. Things like that they promote the use of date rape drugs, physical abuse, blackmail, sexual assault, etc. I'm sure there are folks who fall under the (very broad) category of PUA who do advocate this, but from my time in the community it's most definitely not the norm.
It plays into "social constructs" about men and women that a lot of people despise: Much of PUA teaching is based upon getting guys to approach women, be more confident, be more sexually aggressive, take the reins, etc. This plays into the pretty standard society belief of dating that "men court women, men are the hunters, men make the first move" etc. Some people despise this and see PUA teachings as reinforcing it.
They hate the idea that "just be yourself" doesn't necessarily work: This sort of plays into the 'self esteem' culture that a lot of people buy into, but a lot of people hate the idea that there are certain archetypal personalities women generally find more attractive than others. This spits in the face of the idea a lot of people have of "oh well if you just be yourself then everything will be all right". All of that sounds great on paper, but to pretend that every guy on the planet would have completely equal success with women if they just "be themselves" is well, BS, and PUAs show this. They take guys and train them to behave in a way that is different than "being themselves", and they become much more successful. Now bear in mind I'm not necessarily saying they are happier, just more successful with women in this case.