Tap to pay was pretty well established when they went with scan to pay, they just hoped the Walmart name was large enough that they could cut out all the middlemen inherent with traditional payment processors and keep a bigger piece of the pie. Specifically they were in a spat with Visa over merchant fees at the time so they wanted an option that allowed them to cut Visa out of the picture. They thought that having their own scan to pay system would allow them to do that, but they eventually reconciled with Visa and got the merchant fees discount they wanted and that aspect of the scan to pay system sorta dropped out of focus.