[corysama]

From what I've heard, it's more of a market pressure.

Imagine you wrote a typical 250 page non-fiction book that is padded out in the typical way. Just before publishing, you get fed up and edit it down to just 50 pages that convey everything without any fluff. Your publisher will tell you you're nuts and insist on publishing the 250 page version. Why?

Imagine you are shopping for a book on a topic. You find 6 such books in the store. 5 are the typical 200-300 page pieces. One 50 pages. They all cost the same. You don't have time or patience to investigate their relative quality. Do you A: assume the 50 page book is a low-effort blog series repackaged in print. Or, B: bet that the 50 page version is a high-effort careful edit? Most people choose A and I'd bet they are almost always correct in current practice.

Tldr: Short books look cheap. Customers assume they are ripoffs and don't buy them.