There's one big gap in Apple Inc.'s mobile payment system Apple Pay: Walmart, the world's largest retailer. The mobile payment technology for the iPhone 6 isn't currently accepted by the company and it’s not expected to be accepted anytime soon.

That’s because Walmart is part of Merchant Customer Exchange, a consortium of a number of retail stores, including CVS Health, Rite Aid and more, which are developing a mobile payment system of their own, called CurrentC. Some of those retailers shut down their near-field communication, or NFC, credit card readers following the launch of Apple Pay, despite supporting the technology for some time in their stores.

Unlike Apple Pay, Google Wallet and Softcard, which utilize wireless NFC technology, CurrentC uses a mobile phone’s built-in camera to scan a QR code. Users can also unlock the app to present a QR code that gets scanned by retailers.

When asked why Apple Pay isn’t accepted by Walmart, the company provided this statement to Business Insider:

“There are certainly a lot of compelling technologies being developed, which is great for the mobile-commerce industry as a whole. Ultimately, what matters is that consumers have a payment option that is widely accepted, secure, and developed with their best interests in mind. MCX member merchants already collectively serve a majority of Americans every day. MCX’s members believe merchants are in the best position to provide a mobile solution because of their deep insights into their customers’ shopping and buying experiences.”

Though Walmart emphasizes that it wants a platform that is developed with customers’ “best interest in mind,” some believe it’s more about the fees credit card companies receive from each transaction.

When former Walmart CEO Lee Scott was asked last year by marketing consultant Ron Shevlin why MCX’s CurrentC would succeed, Scott said: “I don’t know that it will, and I don’t care. As long as Visa suffers.”