From what I've been able to gather, these QR code systems are mostly based off of the WEF's Commonpass digitial identity standard (either directly, or inspired by), which seems to be a graduated version of the ID2020 project (many of the same ideas and sponsors).

The projects are meant to present a standardized format for the provision of easily validated information about an individual in environments where low-end hardware is common and internet connectivity is unreliable.

The QR code was chosen as the standard form of information transfer because it can be printed on paper and remain easily validated if someone lacks a device to put it on.

The codes aren't primarily meant to control pandemic spread, this just happens to be the first thing driving their widespread adoption.

Credit cards are widespread (esp in places that bother with covid passes) and where I live something like 90% of payments are contactless. Takes about 5 seconds.

Cynic in me says - just disable cards for the unvacced at specific venues…