Why did JRR Tolkien fail to complete and publish The Silmarillion?

JRRT in retirement - no longer so inwardly driven?

There are many contributory reasons why JRR Tolkien did not complete and publish The Silmarillion in the eighteen-plus years between the publication of The Lord of the Rings and his death in 1973. 

But - over at the Notion Club Papers blog - I describe what I believe was the primary cause of this failure: a decline in his previously genius-characteristic power of sustained and focused motivation (presumably due to age, illnesses, and consequent dwindling vitality).