Earlier quoted context omitted.
2 reasons IMO: 1) We don't have Python3 support yet, we're working towards it, but we're behind some. Have the reference implementation be a regression over the previous one doesn't make much sense. 2) We don't have enough users. And honorable mention reason: RPython is an... interesting language to program, something like a cross between C, Python, Java, with error messages from MUMPS.
> error messages from MUMPS Is fixing that tractable? Cryptic error messages is one of the most frustrating turn-offs to new would-be contributors. /probably one of the few people here who has written MUMPS code
Of all the things that I dislike about MUMPS, error messages aren't anywhere near the top of the list.