They didn’t buy The Silmarillion rights because they aren’t worth anything. Only 1 in 1000 viewers of this show will know who Annatar is, or when Durin’s bane actually awoke, or when the first wizard arrived in Middle Earth. Even fewer will actually care.
LOTR is a far richer story with much more recognizable intellectual property to produce content from. It’s vastly more valuable than The Silmarillion or Unfinished Tales.
Buying the rights to those so some online dorks don’t sperg out over lore inconsistencies would be a totally retarded business decision. Not to mention, they’d sperg out anyway.