That “most” is doing a big lift there. At some point you might consider that you’re actually programming in the language of Pypy and not pure Python. It’s effectively a dialect of the language like Turbo Pascal vs ISO Pascal or RPerl instead of Perl.
Most is more CPython code than python 3 was compatible with. But the port of the broken code was likely much easier than if it had moved to a JIT at the same time too.