Why wasn’t free threading a new version of python?

It was part of the 3.13 release.

Having everything suddenly going from thread safe to C level insecurity is a major change, not just a small feature update.

Sure, which is why it this behavior currently considered experimental and is not enabled by default; if you want to try it out, you have to specifically enable it when building Python from sources.

In fact, if they had to redactor the interpreter to support it, that sounds like even more of a reason to make it a major version update. Doesn’t python follow semver?

No, currently releases follow a regular schedule and every release bumps the second number and keeps the first.