They probably thought about it and then figured it was easier to deploy globally and satisfy regulatory hurdles by working with MC/Visa. Besides, I am almost certain that whatever "solution" will be priced as an extra premium card and that retailers will have greater processing costs that'll conveniently be sent Apple's way.
After all supporting iOS payments says quality, right?
Pair an iWatch with an iPhone/iPad point of sale via iBeacon and you have the simplest payment method of all.