Probably because a company is more likely to use the singular in general sentence construction - whereas the plural is used more as a complete term.
If you look at how the terms are used in the google results you can see the difference, e.g:
Shopping Specials For FNANB Cardholders
Charlottesville Shopping Specials
vs
Luxury Shopping Special Offers, Coupons, Best Deals
InfoSpace Catalog Shopping - Special Offers
World of Shopping : Special Bouquet