I guess because people currently around 17-mid twenties watched it as kids while it was still relatively new, so it brings back a lot of childhood memories for them
I'm convinced it was actually the first proper movie I physically saw. Maybe that's why I'm so obsessed.
If I ever have my own kids I'll make sure TLK (and pretty much all the other Disney movies) are among their first cinematic experiences because I just love them all so much.
^^Though that being said I think people of all ages can enjoy and appreciate TLK for many reasons, like timeless story, memorable characters and beautiful animation :')