To turn up speculation to 100, this might also be a third world issue, because here in the west we have high quality smartphones with good cameras, but the smartphone cameras there might not be as good, so they might be challenged reading QR codes. 8 years ago I built a thing that had customized links accessible via QR codes, but my buddy's cheap phone couldn't read them due to issues with the camera resolution. A lot has happened in 8 years in terms of progress, but they still put crappy cameras into cheaper phones, and this might still pose a problem for reading complex QR codes.
Decreasing the message size while improving security would obviously be ideal and most likely quite achievable, but there are plenty of wealthy municipalities in the US who don't exactly cover themselves in honor in similar situations.
While I'm not at all saying it's illegitimate to speculate on wealth disparities as a cause, in this case I think it's lazy to call this a "third world issue", even with speculation up to 100.
When I was vaccinated, I was given an A4 sheet with those QR codes on it, and I really wouldn't want to scan those with a crappy camera.
[0]: https://gir.st/blog/greenpass.html [1]: https://ec.europa.eu/health/sites/default/files/ehealth/docs... § 4.2.2
Here is the nz version
Longer keylengths make it difficult to deliver sufficient payload in a QR. not sure about EU, but the SMART health passes that are the emerging standard use ES256 signatures.
The lack of global leadership for interoperable standards early on made this more difficult. You had the EU, Israel, US states and others who were ahead of the curve, but that approach had limits that were reached.
Now in the US we also have the issue of dealing with states with wacky political stances. States like California, New York and Louisiana, combined with private sector leaders like Walmart and Epic made SMART the defacto US standard, and other countries are recognizing them.
RSA 512-bits key was proven breakable years ago