1) they don't want to take more of a loss than they have to go get you in the door, 2) the people who buy diet coke or diet Dr pepper are statistically more likely to shop sales than the people who buy non-diet, 3) the relative allotted shelf space and display area aren't just a store manager decision, but actually a product the grocery store sells to the manufacturers, 4) (2) & (3) imply that stores in different class areas of town might have very different which product sells quickly and utterly fail to optimize stocking of the thing you noticed when it literally never sells out at the Kroger a mile away, for example.