- People shopping for Kindle books don't seem to compare books by length the way bookshop buyers do.
- Economies of scale in printing stop being an issue.
- Low pricing of ebooks - if a 200,000 word novel sells at the same price as a 30,000 word novella (e.g. I paid more or less the same price for UR and The Colorado Kid that I paid for Under the Dome), it makes sense for the author to produce shorter, more frequent books.
- Ebooks don't disappear from the shelves as quickly; you don't need to snap them up just in case it goes out of print. So it's in the interest of writers to write books that readers finish, rather than just collect, so that when your next book comes out they're ready to read it.
- Shorter books are less work for everyone involved, so if people can make the same money selling short books that they make selling long ones, they will.
That doesn't mean every book will be shorter, any more than every book is now long - the small press will carry on doing its own thing, as will authors who can set their own terms - but I think these factors will exert a powerful downward pressure on the length of commercial novels over the years to come.
But I could be wrong - we'll see!