The reason for this is simple. Everyone knows what's happening. The major manufacturers are giving huge price breaks to the superstores in many industries, which price breaks are not given to the competing small retailers. The small retailers know this is true because they can buy products in the superstores for less than they are paying the manufacturers. The superstores are often advertising prices for products which are less than the price being paid by the small retailer.
Small retailers know because they are losing customers and are being forced out of business.
Customers know, instinctively, because they are making their purchases, more and more, in the superstores, and not in the small competing retailers. Small wholesalers know this is true, because they are losing their small retailer customers and going out of business themselves. Thus, as to RPA cases, the liability is easily proveable, if not known to most persons. Discovery is generally needed to confirm the obvious about liability, and to disprove the two main defenses, if even asserted at all. Defendants will usually assert the meeting competition defense, in order to have some argument of a defense, but will generally avoid alleging a cost justification defense, because the amount of the combined discounts, fees and allowances is generally far higher than any conceivable cost justified discount, and there is generally no litigation credit for partial cost justification.