I have a couple of theories as to why this Law might occur, and why the occasional exception which proves the rule pops up as well. I open them to discussion:
- The Infinite Monkeys theory -- As everyone knows, an infinite number of monkeys working feverishly on an infinite number of typewriters will, eventually, produce all the great works of Shakespeare. Logically, they will also produce all the great works of science fiction. The corollary is that a sufficiently large number of writers will, over time, produce all the great SF novels that will ever be written, but once such a novel appears, the odds of that particular writer reproducing it is remote at best.
- The Big Bang theory -- Whenever a new science fiction universe pops into existence in the mind (and writing) of a particular SF writer, the number of potentially great novels that can occur in that universe is fixed -- and usually very small. Sometimes that number is zero; often it is between one and two. So, once the first novel is published and revealed to be a fantastic hit, the writer has less than a whole great novel available to him, but because of his contractual obligation to the publisher must write it anyways.
- The Lottery theory -- a fixed number of characters in a fixed number of settings may, against long odds, produce a great story out of themselves. However, once they beat those odds once, the odds of it recurring is remote.