Back in the day it was the best-known and most reliably available cross-platform (iOS, android) code generator - so many sites just assume it as a de facto standard. It also reduces support burden to tell people this is the officially supported code generator, that way you don’t need to struggle understanding why a random app might be having trouble with codes.

These days it’s much less of an issue as there are many more clients - the IdP I manage now mentions authy, tofu and 1password as also supported.