To chime in as a former employee of a text-book shop, whose boss was pretty open about explaining how things worked:
As HarryT mentioned, part of the problem is the small print runs, I'd combine that with a captive market demographic.
In Texas (I don't know about other states), markup on new textbooks is limited by law to 35% (we only did 30% -- which about covered the cost of paying we peons to unpack them, price them, and put them on the shelves) -- 'regular' books are 40% for comparison.
The only ones who really make much money on new
textbooks are the publishers -- who, incidentally, are responsible for the frequent, barely changed new editions: once a certain population of used books gets out into the wild, folks stop buying the new ones altogether, so they adjust it slightly, and release a new one.
The real money for textbook stores are the used books. Think about it, they buy it for 50% and re-sell it for 75% -- a 50% markup. And in a lot of cases they don't even have to put on a new price tag, just stick it back on the shelf, and off you go.
Oddly enough, the student population and the textbook sellers both love used books for exactly the opposite reasons: the sellers because they make more off them, and the buyers because they pay less for them.