Walmart and some other large U.S. retailers were attempting to create their own version of Apple Pay so they were doing their best to not implement a competitor. Last I heard I believe the whole thing fell apart for them.
That was part of it. There's also a significant contingent that are unhappy with the card networks for various reasons (mainly interchange) and don't want to do anything that will possibly encourage even more card use. As it is, cash only very recently dropped below 50% of transactions under $10.
Note: this was all pre-pandemic. The thinking has changed significantly at many of those now thanks to COVID-19.
That was the most annoying thing when I had Samsung Pay. I just wouldn't use it anymore some places because the cashiers were get hostile about it!
I have a Gear S3 watch and never ran into outright hostility. Then again, I found the UX and reliability to be worse than even Google Pay and eventually stopped using it altogether.
And WM is a bunch of idiots saying their own payment system is more convenient. NO it's LESS if I have to use everyone's separate apps instead of just using AP everywhere!
They wouldn't necessarily need everyone using it. If they get enough people on it, then they could start encouraging people to use ACH or some other less expensive forms of payment in-app. If enough people do that, then they potentially have leverage against the card networks that they can use to get better deals (like how PayPal now encourages you to use a CC instead of your bank account after years of doing the opposite).
In addition, more dedicated users of Walmart Pay potentially means more people who will choose Walmart for their future trips instead of one of their competitors, much like Starbucks' app. This assumes payment isn't the only thing the app has, though.