I am in the book publishing business and we do performance how-to automotive titles. There are a few reasons that this isn't more popular.
One is the computer in the garage scenario. Also, books have been around for over 100 years in mass printed form, and CDs are new technology that many people aren't convinced will be here forever.
Also, traditional book outlets and stores don't really carry CDs, and aren't really interested in doing it.
Some of the manuals that come out on CD haven't sold as well as the book version either.
It used to be every year at SEMA 3 or 4 guys would come up to me to try and sell me this service. Doesn't happen so much anymore.