I agree that cost is an issue. Unless they are required, a lot of businesses aren't interested in the added cost.
Somewhat related, since most companies do not buy them, I think a lot of racking reps neglect to mention them when bidding a project. Therefore a lot of warehouse managers may not even know they exist. I hardly ever see them in use in the Midwest, nor do I ever recall a racking rep promoting their use to me.
I should mention that another benefit to the back stops is that they prevent pallets from encroaching on the flue space mandated by fire codes.