Retail shelf space is its own game. Obviously retailers give shelf space to the products that turn the most, and have the most margin. But additionally, suppliers may pay a "slotting fee", or essentially they pay Walmart for the space. Also, suppliers that stock the shelves themselves tend to get more space - more profit for walmart in the end.
So the shelf space game can be as much about retailing, as consumer demand.
Not to mention most people won't use 5W-20 if their manual says 0W-20, even if they live in Florida - so that adds to the confusion.