Audiobooks are expensive to produce, and generally have a smaller market than written copy (whether ebook or dead-tree).
I know at least one author has written about this: http://www.antipope.org/charlie/blog-static/2015/06/cmap-why...
I know at least one author has written about this: http://www.antipope.org/charlie/blog-static/2015/06/cmap-why...
To say nothing of versions where it's just one person reading it straight through, though. Surely those are much cheaper.