Correct. My (wild guess) understanding is that QR codes took off in China because of widespread usage of WeChat, which is a do-all mobile app, including payment

Correct. My (wild guess) understanding is that QR codes took off in China because of widespread usage of WeChat, which is a do-all mobile app, including payments etc. If your QR code requires user to create an account in your webservice or install your native app to do anything useful, it does not bring much value, unless everyone around has your native app. Whereas if you integrate via a an app widely used by all people in the country (i.e. the QR code is a URL which opens the widely used app with certain parameters), you're more likely to have people use it. Classical chicken-and-egg problem and a situation where monopolies do better than a fragmented market.