The store tends to always be there, there is often times need to have a lot of them, and they're available at well known (even published) times whereas the van isn't always at the same places at the same time and doesn't even carry predictable goods... just that you know there will be goods. Sure you can guess or make your luck by waiting for the van or search for it in good spots... but the cost is higher for the criminal to try and count on such a thing.

The truth is I expect stealing from a delivery van is ultimately simpler... or simply stealing the package off the porch easier still. The issue isn't the ease or difficulty.

Where I expect consumer delivery businesses to do better in the face of theft is on the cost of theft (assuming a certain scale in the delivery business). Given the economies of scale of a warehouse and the delivery model vs traditional retail locations, I bet means the loss for any item stolen from the van is less than that of the same item stolen from a traditional retail location.