And as has been pointed out in reference with the Wheel of Time, a reduced sense of obligation. Once the decision is made to NOT end on a third book, you can keep stringing people along forever. Even if the author dies early (not to make light of that per se, or to blame him for dying) or sales eventually fall off because the writing has gotten so bad, and so irrelevant, you have still conned 10 million people into paying 20$, 30$, maybe $50 for this stuff. If combined sales of a series top $100 million for unsatisfying junk, then what is the incentive for the publisher to find some brilliantly written, but short and controversial (AND unorthodox) story, that won't even hit best seller lists, let alone break $1 million in sales? They do not want quality, they don't care if in 20 or 30 years a book is considered a classic, and a revolutionary writing - they're making decisions to make themselves wealthy NOW, and nothing else, it's just business.
I can't believe this is still an argument.
(Also I can't believe I went to do something else, and this post sat here unsent for 5 or 6 hours. O.o)