It must be one of the top ten rules of customer service. Never expect that customer to 'read' anything. And that's not a jab at anyone, its just the nature of sales and customer service. Even if you make every effort to draw attention to something prior to purchase, you will have customers who will buy the product without checking the info first. Its actually, in its own way, a form of compliement. The customer wanted the product so much, they didn't stop to read the product info first. I certainly do that. Bang, new shadowrun product, straight in the cart. I'm not checking authors, page count, file size etc until... perhaps... after I begin reading the product.
But it could be other things. DrivethruRPG has the product info in a column on the left of the product summary. Its visually seperate from the product, enough so anyway that its easy to ignore. So perhaps the information is not presented well, from an Accessability stand-point.
So it could be a combination of things...