Visa and MC destroyed the debit card industry when they came out with their own cards. The biggest impediment to the development of the secondary rails (Cirrus, Maestro et al.) was that card users had to pay a fee to use the card. When Visa came out with the Visa CheckCard it ran on the existing network, and rather than the customer paying the fee the merchant did -- in some cases as high as 2.1% even for brick-and-mortar stores.
WM was at a 1.61% discount rate and tried to negotiate a better rate; MC told them to piss up a rope so WM stopped accepting the debit cards. There as a lawsuit. Lots of ugly behind the scenes stuff (I saw some of it firsthand; very ugly shit given the relatively minor nature of the dispute.)
Just to let you know it wasn't some arbitrary, overnight decision on the part of WM. I hope MC doesn't miss that 1.61% of however many hundreds of millions of dollars a year used to get put on MC debit cards too badly.
Phaedrus