It’s because Walmart has a major, vested interest in people using Walmart Pay, their own little payment application, which their payment terminals support using in a contact-less method. If you try to use Android Pay* or Apple Pay or contact-less cards at those same terminals, they won’t work as the contactless hardware is disabled or not there.
Walmart also goes out of their way to ensure workarounds, like what Samsung phones do with MST, don’t work. Even if the reader accepts your MST workaround, Walmart will refuse to process the payment. Samsung phones supporting MST allow you to hold the phone up to the Magstrip and send the reader your payment info using the Magstrip sensor, which was a pretty cool workaround in cases where a terminal didn’t support contactless payments/Apple Pay/etc.
EDIT: Yes, for a while Samsung Pay via MST worked great at Walmart when you loaded in your average Visa/Mastercard Debit or Credit card. Then it just stopped, while the feature continues to work at other locations.