According to the Tolkien Professor (during his YouTube streams on the History of Middle Earth series) there was always the intent to publish the Quenta Silmarillion (the central tale of the Silmarils) as a First-Age story of the Elves, but it kept getting revised and rewritten and never reached a publishable form.

Until Tolkien's son wanted to complete that piece of the legacy, and found multiple (sometimes contradictory) sets of notes and mostly-finished stories, and Editorial Decisions had to be made.