The used textbook market was on shaky ground well before ebooks arrived on the scene. Every year, publishers force new editions on the market, making changes to the exercises (so old editions are incompatible with new ones) but making no substantial changes to the actual learning material. Some instructors prefer to assign old editions so their students can find cheaper copies, but most don't care, and others are even contractually obligated (via the school) to use only the newest edition.