It's hard to get new items approved, because it takes up store space.
By using the old product, they do none of this and the new version gets into the stores much quicker and into a lot more retail outlets w/o much problem or work or convinicing.
one of the tricks of the trade.
But in many instances, they are relabeling, designing new bottles, new artwork, new marketing material, new "point of purchase" material for the distributors to pass on to the retailers, and reeducating the distributor. This isn't necessarily easier, or inexpensive. And, wouldn't the UPC change as well, (maybe not) to reflect the difference? Some stores do have proof listed on their shelf pricing.