Actual answer is just too many things to quantify, it's a strange style of game outsourced to another team that's known for making a shitty joke mod for Postal 2 and that's it
But the other reason everyone talks about it but never says why is just that it's a meme to do it
"HAHA POSTAL 3 BAD" is the Postal fanbase equivalent of "Pineapple on pizza bad" you're supposed to just say it and get applause and the devs love it because they're not involved and can distract people from the fact that Postal 4 is pretty shoddy also