I was asked, "How did authors like Michael Moorcock maintain the appeal of shorter novels even when they had the option to write longer ones?

At the time Moorcock was writing his many fantasy and Sci-fi novels, most lasting around 150 pages, many other authors were doing the same. Look at Clarkes “A fall of Moondust” or “Rama”. Asimov's, “Foundation”. Poul Anderson, Ursula le Guinn, Alan Gardner, Fritz Leiber and many others were following suit. It was simply the nature of the time. Pulp fiction sold. Keep the cost of the paperback down. Kids (I was one at the time) could not afford to pay for large hardbacks. But pocket money, your paper or milk round would allow you to buy the new Moorcock, Asimov, Clarke or Flint at one hundred and fifty pages or so. If you wished to read Tolstoy, Tolkien or Sarte you went to the library.

After the depredations in the UK of the early seventies. When times began grow a little more stable, a kid no longer, a teenager, Working in a dairy, A freezer shop. I could afford to read longer works and so did. Luminaries such as those mentioned were already writing longer novels, perhaps in anticipation. Moorcock had Just produced “The Laughter of Carthage”, Asimov, “Foundations Edge”

I know this is a weird answer to your question and I may be wildly wrong but I suspect it was simply the financial necessity of the time.