I believe Pennzoil (SOPUS, really) has very tight marketing budgets. This means they can't afford to pay for extra shelf space at major retailers to display their new products (such as PU 5w-40). Shelf space is limited, so whoever (Mobil, Castrol) pays more for it gets it.
An alternative would be to pull some of the existing products and replace them with these new Pennzoil products, but grades such as 5w-40 or 0w-30 are considered niche products, so putting them on the shelf instead of more common (and higher selling) 5w-30 or 5w-20 grades just doesn't make any business sense.