Simply put, nobody wants to shake the boat.
It's like that throughout the industry. Whether its mp3, video, books ... even the PC industry itself. The incumbent players would rather maintain the status quo than have change ... unless they are the ones who change it. They are afraid the change means new rules which may change their present status.
I believe there are some ebooks believer amongst the pbook publishing houses and they may be trying hard to beat the status quo and make ebooks more mainstream. But there are more at stake to all the other players than not. So unless one of these guys get a break and succeed, forcing the other major players to follow suit, it would be hard for ebooks to make a dent in the US pbook arena.
Either that or an existing ebook house manage to pull an "stevie" on the industry and convince enough players to use their distro model or something.
hey, on hindsight, this sounds like a good material for an ebook novel huh?