The question that supplied the title for this thread
begged more than one question. It assumed that there are numerous ("so many") great authors who are neglected.
• What does "many" mean in this context? When I used to teach freshman composition, I had to get after students who used "many" when they couldn't come up with more than two or three (causes, benefits, or whatever)!
• What does "great" mean in this context? Unfortunately, though it hasn't happened on this particular thread, too often when someone refers to a "great" author, after a little discussion someone will assert that "greatness" is just a matter of subjective preference or popularity, etc., yada yada, i.e. there really is no such thing. There
is such a thing. It is possible to use "great" in such a way that the word has meaningful content and, accordingly, can be used in discussion. Is
Paradise Lost a great poem or not? C. S. Lewis thought so, T. S. Eliot (if I'm not mistaken) didn't, at least for a time. There could be a worthwhile discussion of the idea because "poetic greatness" was not a merely emotive term. These readers could have stated for us qualities that they thought a poem, or an author, had to possess, in order to be "great," and these were more than simply matters of personal preference.
• What does "neglected" mean in this context? This was never explained. "Neglected" could mean
---allowed to remain out of print for a sufficient period of time that the author's works were hard to come by and, so, were not finding the readers who would enjoy them
---in print, but overlooked or ignored by purchasers and readers for some reason that could be discussed, e.g. terrible cover art
---absent from discussions where the work(s) ought to be mentioned; for example, if a history of modern fantasy fiction in English was released that had nothing to say about Ursula Le Guin's Earthsea books, we would say that that history had neglected these important (perhaps even "great") works -- although, as long as they were being read anyway, I myself would hesitate to say the Earthsea books "are neglected"
At any rate, it does seem like it's time to come to grips with the assumptions of the thread question and decide whether or not it's been phrased appropriately, or might, rather, have been better, though more cumbersomely, phrased:
Are There Any Great Authors of SF, Fantasy, or Horror Whose Works Are Neglected? If So, Who, and Why?
Does anyone want to nominate an author, or a work, that the reader really does think is (a) great and (b) truly neglected?
I'm not sure there is even one.
I'm not sure that Edgar Pangborn is a
great genre author. But, in any event, when I went looking on Amazon, it appeared to me that his major works are all in print.
https://www.amazon.com/gp/product/0575116927/?tag=brite0a7-20
So -- can someone name even one great, but neglected, genre author?