I believe the story is that the original three Lord of The Rings films were licensed under an old film option deal (as in the option predated Peter Jackson working on the project). That deal was made when the Tolkien estate was under different management or something. And the current regime is not doing any film licensing for Silmarillion or anything else.
I think this is also related to the Rings of Power Amazon series. I think they sublicensed the rights to the LOTR appendices from the same movie option that the Peter Jackson films were made from, but they couldn't get access to a Silmarillion.