Or it could be the author's wish since the unpublished material may contain sensitive information. TS Eliot most famously had an embargo on publishing his letters 50 years after his death or the death of his 'mistress' depending on whoever died later.
Salinger's literary reputation is already set. Don't think any new material will add to or detract from it.
it's about timing, letting the culture soak up the ideas, then release at the time of biggest likelihood of great profit...
consider why has disney+ removed so much content? so they can time it, to sell it at a higher value in the future, once the nostalgia has set in....?
that or they're just too stupid to understand how culture works and are destroying their own value? both of these completely unlikely. but I'm just ranting/guessing online....