I have noticed that although there are many professional (e.g. O'Reilly) books available as PDF, that many textbook publishers choose not to make PDF format available for purchase. What are the reasons behind this. e.g. is it because the PDF format does not offer strong enough copy(right) protection?
Technically, copyright protection is the same for a hard copy book as it is for a book in PDF format. "Copy Protection" is another matter. It's easy for one person to buy a book in PDF format and make a copy of it for 10 classmates. Each of the copied PDF books would be indistinguishable from the original purchased PDF.
PDF files do not have a "permanent" way to identify the "Owner/Purchaser". They would need to be registered, and encrypted and password protected so if the identifying information is modified, the book would not be readable. This sort of information can be embedded into the PDF file but, PDF's can be "deconstructed", and then "re-constructed" with new or false identification information.
At some point in the future, you might see textbooks in electronic form, but they would be a different (not PDF) format.
Either way, that means creating a "custom" PDF (or other) file for each purchaser at the time of sale.
For a printed book, it is much harder. Not many students would want to show up to class with a textbook in photocopied form.
Also, it's about the money.
These days, textbooks can run from $75 to $100 and more. The publishers would have a hard time justifying charging that much for a PDF version.
A non-textbook printed book, say a novel, or a cookbook, only has to be written once. Then it can be printed and reprinted many times and can be sold for decades or longer with little editing or revision.
Textbooks are different. It costs publishers a lot of money to research and pay writers to create textbooks, which quickly become obsolete. A textbook continues to be used and purchased new from publishers for a relatively short period of time. Probably an average of about 5 years max. Depending on the topic, some may last 10 years but that would be rare. Some are obsolete in 2 to 3 years, and some probably quicker than that.
Plus, schools probably don't like the PDF's because they wouldn't be able to profit near as much from book sales, and from used book sales.