There’s real weight in deciding whether a change deserves a bump in the major number of a version tag. “Sure my change is technically backwards incompatible, but who am I to choose whether to move to version 2 of a (private) package? I should just play it safe and bump the minor version to 19.”
I guess part of this is simply because it hasn’t been done already. If the major version number is 1, then there’s real weight to bump it to 2, as you’d be the first one to do so. But if it’s already 2, that seal has already been broken, and there’s much less weight to bumping it again. So I guess conformity strikes again.