Over at Mysterious Matters, mysterious wrote a post recently in which he says that, no matter what they say, more readers (at least of genre fiction) don't want their characters to develop.
I can't speak to mystery fiction -- I don't read much of it, and I don't write mysteries -- but I wonder if that's a problem with series fiction moreso than genre fiction. If you're writing a trilogy or something, I think it would be a good idea for the characters to develop, but I can't imagine what you would have to say about one character over the course of ten or fifteen or twenty books. It seems...impossible.
So if I were a writer of a series like that, I wouldn't work very hard on character development. I wouldn't want my characters to develop very much. Mysterious points out that in Agatha Christie the detectives didn't develop because they weren't supposed to--the focus of the reader was on the mystery and the puzzle, and the detective was just a gateway for the reader to enter into the story. Maybe the problem now, then, isn't that the characters in long-running series don't develop, but that readers expect them to.
And maybe that's why I don't read many mystery series -- because I do want my characters to develop and change and grow and be different by the end and that's just not a realistic expectation when you're talking a bazillion books.