I think the problem was that Robert Jordan started getting bored with it. Book 1-3, great. Book 4 and 5, needed story exposition, back characters, etc, a bit long, but I get it. You needed something like that to give the story depth. Book 5, he has his big heart attack. After that, his writing changed. Changed for the longer, but not necessarily for the better. Books 6-8 realistically could have been 1 book. Book 9, when they finally do the really cool thing they have been hinting at for 8 books, that was worth it. But then he goes back to "i'm writing because I get paid by the word" mode. And then after putting everyoine through that, he dies before finishing it. Though I will say that Branden Sanderson really seems to be doing a good interpertation to finish the series, much closer to the first 3 books, and perhaps even a bit better.
I figure George R.R. martin is going to end up doing the same thing to us.