Note that the money you can make out of the vast majority of academic textbooks is so low that it couldn't be a strong motivation for writing textbooks.

While I am usually reluctant with labeling questions as following the "boat programming" scheme, I am inclined to think answers to this questions are not in any way different concerning authors from academia compared to other authors. On top of that, note that a book about programming is not necessarily an academic textbook, at least not just because it has been written by someone working in academia.

If one sees one's goals as education of others and diffusion of ideas, it makes sense to make lecture notes and books as widely available as possible at as low a cost as is possible.

@MassimoOrtolano How much (or little) money do authors make from academic textbooks? My guesstimate is that if one copy of the textbook sells for say US$ 200, and the author earns say $5 per copy, and sells 1000 copies a year, that's $5000 a year which is a reasonable amount.

Have you heard about the Open Software community? :P

@DanielArmengod Yes, I'm quite familiar with open source software. You're right that there is some similarities between open source software and free textbooks.

@ILiketoCode Books don't sell themselves. They need money to advertise them. That's what publishers do, as well as actually producing the physical objects. From a publisher's point of view, "yet another introduction to computer programming using Java" is something that is competing with hundreds of similar books already published, and it isn't going to make any significant money for the publisher with sales of "only" 1000 copies per year - so they are unlikely to be interested in it.

Some more examples: The Elements of Statistical Learning and Deep Learning are both available for free and regarded as top textbooks in their fields.

It should be noted that academics asking this question for free are not very likely to be surprised by others academics publishing books for free.

See also this question: academia.stackexchange.com/q/63619/20058

Why don't you email him and ask; math.hws.edu/eck

See also The Case for Free Online Books (FOBs): Experiences with "Operating Systems: Three Easy Pieces", linked from the book's webpage as “Why Textbooks Should Be Free”.

Why do people publish research papers for free? Or worse, often pay to publish them and present them at conferences for free consumption by others?

It is a cultural thing, too. It is much more easy to find free (legally, not pirated) textbooks and information about programming, than sciences. Maybe it is result of open source culture, or setting up your server, putting your content on it is much more obvious thing to do, if you spend more time in IT, than if you are eg a Chemist or Biologist.

Well, WHY NOT??? I love writing books, not getting paid for that, and seeing everyone either ignoring it or scolding the typos in it ;-).

I have found that almost always, authors of academic textbooks prefer to make money from sales of their textbooks, rather than writing a textbook which is available freely without cost. -- please cite your sources

Why would an academic spend his time on Stack Exchange helping others for free? Why does open source exist? Why do charities exist? Volunteer workers? Interns?

@DanEsparza I don't have a source, but my (anecdotal) personal experience suggests that non-open source textbooks from academic publishers are most commonly prescribed in courses.