Well, it’s probably quite the combination of different things
Accepting card payments through PoS requires a terminal
Alipay and WeChat? QR based. Can be done through a phone, or anything else under the sun.
Not sure about the fees but Alibaba and Tencent are so ingrained into Chinese daily life that they could make a monopoly out of anything if they integrate it with one of their apps.
People love convenience. This is why Google and Apple Pay are so popular. They’re provided by companies you know are trustworthy and that you know will work reliably if your phone supports it.
In a market like China where low-end phones sell incredibly fast and where Google isn’t allowed to sell things? There’s no ultimate reliability with NFC access on phones (unlike here where it’s literally a given your phone has NFC, although my friends phone doesn’t support it he has a Chinese phone!)
So QR code is about as convenient as it can get, it works on all your stuff. It works P2P. It’s secure (data is only stored on your device and you read the merchant’s data).
Again, I’m not sure about fees, but just imagine that tomorrow Facebook Messenger introduces FBMP and it pays Tesco, Lidl, Aldi, Asda, Shell, BP and all your other day-to-day merchants a nice down payment to take their payment method
The next day they all have it and the acceptance is good enough that it’s convenient