tolkien was very attached to the silmarillion but could never get publishers to pick it up. he actually briefly parted ways with allen and unwin to see if another british publisher would give the silmarillion a shot, but when that didn't happen he came back to A&U with snippets of what would become LotR, and the rest is history. the problem for tolkien was that the silmarillion never stopped evolving, to the point that no coherent drafts could be assembled during his lifetime (christopher tolkien's efforts compiling what we have today were nothing short of herculean if you read through histories of middle earth and track the numerous changes), and the problem for potential readers was that the scale of the thing was just immense and completely unlike anything else at the time.
