Well, different people have different taste. Just because you don't like it, it gives you the right to bash people who do.
I think it's more because it has a memorizing quality that most similar manga don't have. I hate horror and gore, to the point where I will actively avoid it, but it just got me hooked. And I know a lot of people in the fandom didn't originally like horror either, so for us, this is all new. It's like watching a car crash, you're really scared and you don't like it, but you can't stop watching.
I think also, we've seen the stalker storyline, and the serial killer storyline, but usually not together. Having a Stockholm story where the victim was already obsessive with the captor before the story even starts, feels very new. So in essence, it's more about the captor developing Lima syndrome and whether or not his condition is enough to keep the victim safe from further mental and physical torture. Most Stockholm stories don't have a serious Lima element to them. It's usually the captor playing around with the victim without much real investment if the captor doesn't do a complete 180 and just becoming nice. In this story, while the captor has shown compassion and affection towards the victim, he's made it perfectly clear that he's willing to continue torturing his beloved. That's the element that everyone likes to analyse.
I don't get why Killing Stalking is so popular besides the fact that the YOI fandom has apparently moved there. Like tbh to me it's meh. Like I've seen it before so it's not really that great. People think KS is fucked up. Bruh have you read Harada's work?