Why do bookstores usually only sell 'How to Play' books/CDs/DVDs for guitar, piano, or harmonica?

Demand, that's all.

General bookstores will stock the books that they think people who come there want to buy. Many more people play (or want to play) piano, guitar, bass, and drums than want to play viola, oboe, or cello (for example) and therefor there's a demand for some books and not so much for others.

Music stores, though, attract a specific clientele that just happens to play (or want to play) musical instruments. It's there that you'll find a demand for books/cds/dvds about all sorts of other instruments. It's also the place you'll find those instruments for sale. The selection available in a music store is much more diverse.

You'll also find that the scope of materials for those more popular instruments is greater in a music store than in your general bookseller.

First, there are dozens of popular violin instruction books available at any violin or music store, but your assumption that violins are inexpensive is wrong. Most guitars and certainly harmonica's are cheaper than violins, at least ones that are worth buying. Yes, you can buy a violin for $60.00, but they are absolute junk. Violins worth buying cost at least $300, whereas beginning guitars that produce a reasonable sound run around $150. Because of the way guitars are made, they are much cheaper to produce than violins.

Of course, Piano's are a complete different story.

Guitars and Harmonicas are much easier to learn on your own and even piano, where the note is the note. On violin, you have to learn to hear early on so that you can play in tune. Using finger position markers on the violin helps, but it is only a crutch that can slow down the proper learning of the instrument. The violin is much easier to learn with a teacher.