This is because the digital book industry is in transition and book publishers have not yet decided how to handle digital library lending. In fact, a number of big publishers won't sell their e-books to libraries at all! Others choose not sell their e-books to OverDrive, the software vendor that powers the Library2Go service.
Ursula recently wrote a blog post on e-books and libraries, with some interesting factoids about the current state of the industry. It's a short and interesting read and provides a little more insight into why we can't always get the e-books you'd like to see in Library2Go.
WCCLS has signed on to ReadersFirst, a movement to improve e-book access and services to library users.