That's because nginx doesn't break things for end user every release, so there is no reason to bump major version.

I bet nginx doesn't even follow semantic versioning, which you seem to be talking about.

Don't have to bet: Nginx doesn't follow it. It has its own linux-kernel (odd vs evens) inspired convention.

Doesn't change the fact that only "breaking" changes in 1.x.x line are changes to defaults.