Why can't Python 3.3 be compiled for it? Even if python.org don't go and do it, it's entirely possible the build system changes required could be backported.
That's a technical detail which probably one of the core developers might have a better answer (I don't use a Mac and am not aware of how Apple Silicon Builds generally work).
As to dumping 3.3 and solely relying on 3.8 (not 3.9), I don't think the developers think it worthwhile to dump an entire package ecosystem built in the last 10 years just for a native build.
Just upgrading to 3.8 would drop support for a fair number of plugins - python is only somewhat source backwards compatible and not at all binary backwards compatible. As for why 3.3 is difficult to compile for macOS ARM, the 3.8 port is actually a fairly large changeset that would need to be back-ported and tested for 3.3.
Are the plugins loaded in to the main ST process (using an embedded interpreter)?
A subprocess plugin architecture that used IPC to talk to the text editor would solve the problem, since the plugin host process could probably remain x86 and run under Rosetta.
As to dumping 3.3 and solely relying on 3.8 (not 3.9), I don't think the developers think it worthwhile to dump an entire package ecosystem built in the last 10 years just for a native build.