I'm an alien visitor to the US market. Here's how my editor explained it to me:
Until the early 1990s, mass market SF/F paperbacks were primarily sold via grocery store racks, supplied by local distributors (400+ of them).
During the inflationary 1970s and early 1980s, the publishers wanted to increase their cover prices. But the grocery wholesalers who sold the books insisted "the product's gotta weigh more if you want to charge more". So just as buffalo tomatoes got bigger, so did paperbacks. But you can only get so much milage by using thicker paper and a bigger typeface.
In the 1960s, an SF novel was 60-80,000 words, with 80K being considered overblown and long. By 1990 they'd grown to 90-100,000 words.
Then in 1992 or thereabouts Walmart woke up and said "why the heck are we using eighty bazillion distributors?" and fired 90% of them. They went from 40 in California to just 2. The mass market book racks imploded as a sales channel. But that left Barnes and Noble and Borders a market vacuum to fill. So all was well for a while, with the midlist paperback market replaced by a midlist hardcover market.
Same pressure applies: publishers want to get more money per book, so they try to make the hardbacks bigger. Finally, circa 2001, Borders yanked the brake handle and said "we won't buy any non-bestselling titles that cost over $24 in hardcover or $7 in mass market -- they don't shift" (each $1 over $24 cut sales turnover by 20%, IIRC).
Anyway.
We in SF/F have been trained to expect longer books by the grocery distributors.
I would hypothesize that mysteries did not succumb to this selection pressure because there's a countervailing force at work -- the reader's ability to keep track of multiple characters and plot threads. This is aggravated by the gritty, terse style that has been de rigeur in mystery since the day of Raymond Chandler or Damon Runyon -- yes, there are exceptions, but florid verbosity is generally frowned upon, so you don't get the purple passages so typical of a certain type of fantasy.
The way David Hartwell explained it to me ...
Until the early 1990s, mass market SF/F paperbacks were primarily sold via grocery store racks, supplied by local distributors (400+ of them).
During the inflationary 1970s and early 1980s, the publishers wanted to increase their cover prices. But the grocery wholesalers who sold the books insisted "the product's gotta weigh more if you want to charge more". So just as buffalo tomatoes got bigger, so did paperbacks. But you can only get so much milage by using thicker paper and a bigger typeface.
In the 1960s, an SF novel was 60-80,000 words, with 80K being considered overblown and long. By 1990 they'd grown to 90-100,000 words.
Then in 1992 or thereabouts Walmart woke up and said "why the heck are we using eighty bazillion distributors?" and fired 90% of them. They went from 40 in California to just 2. The mass market book racks imploded as a sales channel. But that left Barnes and Noble and Borders a market vacuum to fill. So all was well for a while, with the midlist paperback market replaced by a midlist hardcover market.
Same pressure applies: publishers want to get more money per book, so they try to make the hardbacks bigger. Finally, circa 2001, Borders yanked the brake handle and said "we won't buy any non-bestselling titles that cost over $24 in hardcover or $7 in mass market -- they don't shift" (each $1 over $24 cut sales turnover by 20%, IIRC).
Anyway.
We in SF/F have been trained to expect longer books by the grocery distributors.
I would hypothesize that mysteries did not succumb to this selection pressure because there's a countervailing force at work -- the reader's ability to keep track of multiple characters and plot threads. This is aggravated by the gritty, terse style that has been de rigeur in mystery since the day of Raymond Chandler or Damon Runyon -- yes, there are exceptions, but florid verbosity is generally frowned upon, so you don't get the purple passages so typical of a certain type of fantasy.