It has nothing to do with the status of the characters, or anything like that. It’s that “Rye” was always an anti-classic, so to speak. Adults don’t get it, man. It can only be “relevant” to one generation at a time. What did it mean to be a catcher in the rye, do you remember? Holden Caulfield wanted to save kids from losing their whateverness in the grownup world. Something like that can’t become a dusty, old library book that your grandparents enjoyed.
Classics are supposed to be perennially relevant. That “Catcher” doesn’t feel right being forced on subsequent generations is a good indication it was never destined to be a classic. It will remain primarily a baby boomer thing, I expect. No matter, there are more good entries in the bildungsroman genre than you could ever read in one lifetime, probably.