It is a common occurrence in American publishing that if an author has a big hit, publishers push for a sequel.  If that also does well they push for another book to make a trilogy.  If that flies they push even harder to make a series and then they ride the series until they wear it out.  I suspect that is what happened to Herbert.  The ending of the third book where the main character (son or grandson of Muad'Dib?) basically wins all the chips and issues the stern prophesy about a regime that will last for a thousand years and that's the end of that, seemed to me to be a not so subtle piss-off to his publishers.  I took him at his word and didn't bother reading any of the others.

Now where are we going and what's with the handbasket?