It is an interesting question why JRR Tolkien did not finish and publish The Silmarillion; despite that The Lord of the Rings was published in late 1955 and Tolkien did not die until mid-1973; and despite that all through these seventeen-plus years he was insistent that making The Silmarillion publishable was his number one priority.
Much has been written, and truly enough, about the many technical difficulties facing Tolkien in this work, and about his habits of making matters harder for himself by writing new material (and radically changing old material) rather than focusing on the core task of making The Silmarillion both internally consist, and consistent with the published Lord of the Rings.
I would like to take a different approach altogether. This recognizes that JRR Tolkien was a genius, The Lord of the Rings was a work of genius, and Tolkien wanted The Silmarillion also to be a work of genius.
By my understanding of the nature of genius; to do work of genius requires both exceptionally high ability, and also exceptionally strong motivation.
This exceptional motivation comes from within (i.e. is "endogenous" - inner-generated) - and can only come from within. The motivation is focused to accomplish the particular task a genius feel inwardly compelled to accomplish.
Such a powerful and sustained inner motivation is something present... or not.
Because of the difficulty of accomplishing a work of genius, only a genuine and inner motivation will suffice to provide the sustained and directed energies and concentration that is necessary.
Self-exhortation, external pressures, the sense of duty, potential financial or other benefits... these things can be no substitute for a genius's special capacity for inner motivation, directed to do what he "must".
The Letters of JRR Tolkien document the power and resilience of his motivation to write the Lord of the Rings (LotR), extending over more than a decade - and persisting despite many obstacles such as multiple urgent and competing duties at home and in work, World War Two, physical and psychological illness; and his innate reluctance to finish any piece of work, but instead to "niggle" at it.
Tolkien wrote and published Lord of the Rings; despite several longish hiatuses and interruptions - some self-inflicted; but always he would return to the job, again and again, pushing forward over-and-over with the massive task - even as its difficulties continued to expand.
We need to recognize the colossal expenditure of directed and sustained effort that this required - Tolkien often emphasized this himself in later writings, including the Foreword to Lord of the Rings.
My point is that genius-level writing is not only a product of genius-level ability; but also genius-level motivation.
It was primarily a decline of motivation that failed Tolkien in his intention to complete and publish The Silmarillion - at a level of attainment commensurate with the earlier published book.
Such a decline in motivation was, of course, not Tolkien's "fault". Indeed, it is more accurate to put things the other way around...
More accurate to say that Tolkien was temporarily blessed with a tremendous power of self-motivation during the years of his late middle-age; carrying him through from the beginnings of LotR in 1937, at least to submission of its final volume in 1955 - by which time Tolkien was sixty-three years old.
The (apparent) fact that this extraordinary motivation was not sustained over the entire remainder of his life and through into extreme old-age; is simply that such things depend on our mortal and vulnerable bodies and minds; therefore they don't usually last very long.
I think his ability as a writer remained largely intact; in that some of his very best pieces of writing came after the publication of Lord of the Rings and up to the late 1960s. these including Silmarillion themed pieces; posthumously published in Unfinished Tales and The History of Middle earth.
So I believe it was diminished motivation rather diminished ability that was the problem with The Silmarillion.
Tolkien was to some extent unable, and also to some extent unwilling, to focus his motivation on The Silmarillion. This is evident in the memoir from Clyde Kilby, about assisting Tolkien in the middle 1960s - ostensibly to help him complete The Silmarillion.
Kilby reports that (on a day by day basis) Tolkien went to considerable lengths to do almost-anything-else, other than work on The Silmarillion!
But a particularly telling example is documented in the published Letters.
Immediately after the publication of LotR; Tolkien claimed, in many letters, to be unable to focus upon The Silmarillion because he needed to complete several philological publishing commitments, as well as continue the duties related to his position as a Professor of English at Oxford.
However he was due to retire in just a couple of years...
But in 1957 Tolkien arranged to stay on as Professor for an extra two years. This he stated to be due to money worries, related to a modest pension. In the event (as Tolkien realized by 1958) there was no reason to be concerned about his income, because income from royalties on LotR soon proved to be very substantial indeed.
In correspondence; Tolkien said he wished he had known this before taking on the extra two years as Professor - which was not, after all, proving to be financially helpful (due to the effect on taxation of his royalty income). Tolkien said that if he had known about the extra LotR income, he would have retired earlier, at the statutory age.
Tolkien finally retired in 1959, aged 67 - this was now four years post-LotR.
However, in a letter dated 21st July 1962; Tolkien announced that from October 10th "I have to stand in for the outgoing Professor of Anglo Saxon" until the new Professor took up his post in Easter 1963.
Of course; Tolkien did not "have to" do anything of the sort! It was Tolkien's choice - a matter of his priorities, not his duties; and there was no financial necessity.
Tolkien was by 1962 now seventy one years old, and had been fully retired for three years; and was five years post his official retirement - and it was now fully seven years since the Lord of the Rings had been published. Seven years since he was supposed to be concentrating his best energies on completing and publishing The Silmarillion...
Yet, here was Tolkien voluntarily returning to university duties that had stood in the way of his writing for decades, the onerous nature of which he had very frequently complained in his earlier letters!
What I infer from this is that, in his later life - approximately the final eighteen years, Tolkien lacked the colossal drive necessary to making The Silmarillion his priority, setting aside other claims on his time and energies, and pushing through to its completion.
It was this inner lack of motivation, presumably due to increasing age and diminishing vitality - rather than anything else; that was critical in Tolkien's failure to finish The Silmarillion.