Simple enough really, he never finished it but like most people with a dream he's probably trying ever now and again, perhaps even getting bits of it done (seeing as it was shown he has some talent for it but never got the chance for anyone to tell him that).
Ending worked for me. It wasn't as bad as it could have been in some respect and yeah it was kind of bland when you consider how awesome some of the twists and turns were throughout, but like others said they probably had to appease the publishers.
For the better really, I always find myself in deep thought after some of the chapters. There are often parts that really know where to get you from experience.