My guess is that promo codes are used for fares sales to prevent stacking of other promo codes that Southwest gives out. Such as the one I got before the holidays for viewing an Apple booth at OAK. If these could be stacked on top of a 35% off sale, there would be some screaming deals possible.

Southwest must have a tough time creating a new fare class for a sale and populating it with seats. That's what they'd have to do to prevent stacking of other promo codes.