You should see the lineup of what it is like to suck air in other subjects! No joke.
First, let me say that what I am about to list does not mean that there aren't books in these areas which aren't good, probably some of them are. But they lack a couple features: marketability (an audience to target) and a reason to exist. By "reason" I mean other than a University Press cranking out stuff which hires editors, printers, proofers, designers... all people I love in those professions, but for which an audience of any valuable size is non existent. In other words, trees are suffering and so is the environment because of the plenitude of books on stuff few care about. These should be done (if at all, and I would vote thumbs down on 80% of them) in small press private quantities instead of enveloping warehouses and waiting for a landfill burial.
Some are reasonably priced (?), some are in "this will never sell" hardcover.
African-American Studies (job creation, not over national guilt)
History pre 20th century (beating subjects like Lincoln, Jim Crow, etc. into the ground, more books on the Civil War than those who died IN the Civil War!)
Ancient and Classical Studies, Anthropology (you mean all the "old stuff" is really bad?)
Architecture and Art and Graphic Design (the branch of unreality when it comes to making dollars but wanting others to pay for their excesses)
Asian studies (written mostly by non-Asians!)
Books on Books and Map Making (wow, lots of losers here too)
Classical Music and Opera (lots of time on their hands writing instead of performing)
Cookbooks (and yet there are big collectors of these)
Cultural Studies (Colleges love this stuff since you don't have to prove anything)
Economics and Business History (lots of incorrect second-guessers)
Fiction & Poetry (probably the BIGGEST losers on the list... everyone has a story... but not everyone can tell one well)
Film, Television & Theater (another cultural and artistic boondoggle, do not publish these)
Judaica (lots published but the buyers are too cheap? sort of like poetry)
Latin American Studies (the people about whom these are written have no money, so it must go mostly to professors (who often get them free)... Latin American chess books aren't very popular either)
Law (one might expect this)
Literary Studies and Memoirs... probably on a par with fiction (novels). Everyone has an opinion but few want to read others'.
Mathematics and Physics. These often sell well (if they aren't given away) but the quantities printed are often over the top.
Medieval and Renaissance. A time past and missed. Opinions and wishful thinking.
Military History. A major yawner but lots of aficionados.
Philosophy. Isn't this just a scientific sounding name for "opinion?"
Photography. Kept Kodak alive, for a while. Now with digital they are in trouble. Another great art form but way too many.
Political Science. You can tell can't you, in advance, what the losers will be because they have the biggest sections in the stores, the slowest sales, and absolutely no reason for that shelf space. Stuff like this didn't help Borders. Maybe argumentation doesn't sell.
Popular music. Lots of musicians out there. In fact in any "craft" where there are a lot of people employed (gainfully, or otherwise, you can expect to see books--except when it comes to blue collar labor because those guys/gals read books about OTHER stuff!)
Psychology. Well, no surprise here. Statistics coupled with opinions and theories.
Science and Natural History.
Sports and Games. Some of this sells but often are just photographs and stats.
Technology and Computer Science. The "older" it is, the less well it does.
Theology and Religion.
Womens Studies. Worth studying, not so sure about writing about.
ARE THERE ANY WINNERS?
Few books have been remaindered on:
Antiques and Collectibles (ah, making money off of old stuff)
Australian and the Pacific (a mystery to me)
Chess (Questionable, all the remainders are mostly from Cardoza!) Smaller printings?
Children (I think publishers are holding back because there are WAAAY too many of these)
Comics and Graphic Novels (lots of under 40s still buying this stuff)
Dance (my suspicion? few are actually written, but then there is always the merengue!)
Medicine.... Always expensive... Jury is still out.
Occult and New Age. No accounting for taste I guess.
Pop culture and current affairs. The "People" magazine crowd.
Of course there are subjects I didn't mention because the sample size is not easy to determine. But if you've been to many book shows, or remainder shows, and I have, the "losers" mentioned above are in HUGE supply and when you look through them, some of them are quite good, but for one reason or another, price, quantity, audience, marketing... they just didn't do that well EXCEPT for publishers who deliberately overprint and try to recoup their expenses from reselling remainders.
And by the way, just because it has sex in the title doesn't make it a best-seller. When I was in San Francisco many years ago, there was a store whose name I can't recall but they had a big section on gay and lesbian stuff. But, NO ONE was in there reading, buying, gawking, or paying any attention to those books and this was SF!! When it comes to rallies and complaining they show up, but when it comes to spending cash, that's a different story.
Properly done, marketing can change minds but it can't change real unadulterated facts.