Tolkien abandoned the sequel idea he had for LotR. He didn't just not finish it. He abandoned it.
Were you adopted?
In rode the Lord of the Nazgûl. A great black shape against the fires beyond he loomed up, grown to a vast menace of despair. In rode the Lord of the Nazgûl, under the archway that no enemy ever yet had passed, and all fled before his face. All save one.
I put up a detailed reply to this thread last night just to have it all erased. But I'll give it a go again.
As was discussed in the past on IMDB, the Lord of the Rings, particularly the Fellowship, was made with more care than The Hobbit. The first trilogy (and particularly the first film) weren't guaranteed to be a success, while The Hobbit was a "done deal", just about, based on the former films' reputation. So the same monumental effort, attention to detail, care, affection for the work, caution, etc. weren't put into it; the new films didn't have the sense of serendipity or spontaneity, of uncertainty and vulnerability, of "will we be well received?" that the older films had. The steel determination to go forward despite the odds, almost in spirit with the tale, was lacking.
As BB has mentioned before, this was somewhat less Jackson & Co.'s fault (or Howard Shore's), in many ways, and more the producers' and Warner Brothers' (with New Line Cinema having been acquired by Time Warner in 2008 and Robert Shaye and New Line's former staff having been given the boot). They wanted to milk the new films for all they could and thus made them in a frenzied way and split what were originally Jackson's two Hobbit films into three. This, the CGI overload, etc. didn't make the Hobbit Oscar-worthy material like the former films were. (Even as a "lesser" literary work than the Lord of the Rings, I think The Hobbit could have been more; the book is quite whimsical and has some fine qualities).
If the Silmarillion ever got made, after Christopher Tolkien passed, or any of its stories, I would hope they would be made with more care and reverence like the Lord of the Rings, and not with the "banking on previous films' success", corporate greed and relative carelessness The Hobbit was made with.