Why has it taken so much longer for CPython to get a JIT than, say, PyPy? I would imagine the latter has far less engineering effort and funding put into it.
It's only fairly recently that there's been critical mass of people who thought that performance trumps simplicity, and even then, it's only to a point.
This definitely wasn't true, from the user perspective. And, I'm not even convinced it's some "critical mass" of developers. These changes aren't coming from some mass of developers, there's coming from a few experts that had a clear plan, backed by the sanity of the huge disconnect that languages are actually meant for users of the language, not the developers of the language.
That made it much easier to get bigger gains than if they wanted 100% backwards compatibility with CPython.