Because it costs money to publish a book -- especially one with illustrations -- but is much cheaper to post stuff online. It's not a question of whether it's desirable to publish this info in book form, but whether it's economically possible to do so. There's not much market for Trek reference books in general these days, and the market for some kind of guidebook to a novel-only series would be far too tiny to make it worth the expense of creating and publishing it. The company would lose too much money on the project. But if some of us authors create artwork on our own time and post it online, that's something that can be done essentially for free. So it's the one thing we can do.
