The short answer is materials and convenience. Moving boxes are made to be more rugged often using double wall construction which costs more to produce. While moving we often are more interested in getting boxes quicker and do less comparison shopping. That allows self-storage, truck rentals and retailers to price them according to our lack of planning.
The amount you’ll pay for boxes varies by the quality, quantity and geographic location. Shop around well in advance of your move to get the best deal. Expect to pay in the area of $2 for book boxes, $3.50 for general packing a file boxes, and up to $9 for wardrobe box.
Many moving, self-storage and truck rental companies are now very competitive selling boxes and moving supplies. It still pays to shop around especially now that many companies are offering delivery service of boxes and moving supplies.