http://www.theatlantic.com/business/...ebooks/284486/
A crucial factor seems to be court ruling that says ebooks require separate licensing and the original agreement does not apply to electronic or other format.
The article does not discuss demand. Of course, how much demand is there for books pre 1923, but they are widely available both for free and sale.
PS: A similar legal snag keeps many movies and tv shows from being released on discs. The non original songs used on soundtracks are often licensed only for the movie release. The movie studios prefer to pay nothing, whereas the song copyright holders want to extract as much as they can get. For small movies or documentaries, it's often not worth it for the studios. (Mad Men reportedly paid $250k for using 20secs of a Beatles song. If that covers DVD and streaming, it's a pretty good deal.)