Although the music industry pretty much said goodbye to the audio cassette years ago, once CDs came along, the audio book folks stuck with audio cassettes because they proved a cost-effective alternative to CDs as well as having more than enough audio fidelity for the spoken word.
However, its time has come. Only 400k tapes were sold in 2007, and that number is expected to plummet this year. Audio books on cassettes may still live a few more years as libraries still demand them, but not too much more than that, even though most households still have at least 1 or more cassette players on hand; audio books are also going the way of digital downloads and streams. Many publishers have already completely abandoned cassettes, including some large ones like Random House and Macmillan.
RIP audio cassette – you had your place in history and in my audio collection.