13th? First 10 or 12? Number 9? Isn't it just possible that readers are simply tiring of the same old sh!t after that many books with the same characters/settings? I know I do at about the 3-4 book mark... even with authors I really, really like.
Readers share a big chunk of the responsibility here too. Just because authors are writing series that run to double-digit installments, doesn't mean fans have to continue buying them. If interest wanes and dollars start running thin, they'll wrap up and start something new. If you keep buying, however, there's no reason for them to stop riding the gravy train for all it's worth.