I could be misremembering since I lived in China a long time ago, but I feel like QR codes were wildly popular in Asia long before digital payments became really popular. So it probably made sense on several fronts to adopt them for payments as well (everyone already used them anyway, they're effectively free, etc.) rather than build a new system.
I didn't think they were especially popular until they were used for mobile payments. I'm not really sure about other countries in east Asia, they tend to grow completely different ecosystems from China.
