From an employee I talked to it's because they have a specific contract with the payment processor and it requires using specific payment devices that are covered in the contract and they don't want off it for as long as possible because it gives them preferential fees.
So until the cost of business lost is enough to cover increased payment processing fees, don't expect to see tap to pay.

Do the stores not have free WiFi? I've never been in a Walmart, but all bigger stores here in Europe have WiFi.