That's the big downside. Even though the quality is there, Cengage books are quite expensive - they're like the Apple of the publishing world.
I think the main reasons for this are that they are very polished (3 editors, 5 reviewers, bigger pipeline) and come with a ton of educational resources (PPTs, testbanks, supplementary labs, case studies, research exercises, MindTap online labs/LMS, etc.). Hence why they stick to the academic markets only.
I'm trying to convince Michael Warren Lucas to write a Linux textbook. I don't think he'll venture out of the BSD realm, but if he ever did write a Linux textbook, I'm sure it'd be hilarious (and good)