I think you hit on something very important - that the buyers who decide what gets sent to the bookstore matter a lot.
Let me give you a couple of examples from the days when my parents ran a general and Christian books and music store...
The store was located close to several antique shops - women would go antiquing, drag their husbands along, then stop at the bookstore. Our bookstore had a huge selection of books on antiques and collectibles for just this reason - stuff you would be hard pressed to find even today in a Borders or B&N, like a $200 book on Tiffany Lamps, or even a $30 book on Depression era pressed glass.
Of course, the men were often bored - until they saw the books we had on cars, trucks, car repair, woodworking, military aircraft, etc.
We would sell at least $1000/week of car repair manuals such as the Chilton or Motorcraft series (and this was a small store). The $17 book on small gas engine repair (lawnmowers etc.) did very well also.
When we found people were restoring old Mustangs, we started carrying the $80-$110 Hollander Interchange manuals, which tell you which parts from which Mustangs are exactly the same for each year (e.g. a suspension component from any of 65-68 Mustangs might be identical, making it easier to find the part).
If you walk into a Borders today, you will find very little customization of each store for the local market. For instance, in the York, PA, store, home of a big Harley Davidson factory and filled with people who like hot rods (big cruise nights in the area) ... not even a small table packed with such books.
Second example: the Christian music buyers were always focused on the teen market (Christian Rock like Petra, etc.) -- but Christian music-buying teens usually have far less allowance money (IME) because their parents are more conservative.
So our music selection ignored the teens, and focused on having the music that the parents or other Christian adults would want (tended to be Christian country, Sandi Patti, and Christmas albums). Those folks, being adults, and in the prime of their earning years, had little hesitation in buying, often multiple items at a time. The buyers at other stores turned up their nose at such folksy stuff, to their detriment I think.
Imagine the sales lost by bookstores nationwide by focusing on Twilight books, heavily discounted, which attract distracted and less-rich teens, versus looking for and exploiting local niches.