Charles Stross explains:

Once a trend like that becomes established, it's hard to stop. Put yourself in the position of a bored browser in front of a supermarket wire-rack, contemplating novels by two authors you've never read. They both cost the same, and you have enough pocket money to buy one. The year is 1980...How do you make your mind up? Well, you remember what you've heard about the authors, and you look at the cover painting, and you read the back flap blurb. Assuming all of these are equal ... you probably buy on weight, because you subconsciously anticipate a longer reading experience and, all things considered, good experiences that last longer are better than short ones. Remember that the actual cost of the paper and ink is only a small component of the retail price of a book around 10-15%. Increasing a book block's size from 150 pages to 180 pages is cheap. And so, from the 1960s to the 1990s, publishers unconsciously trained readers to expect longer novels.