I'm under the impression that there's two reasons we don't have it in chromium yet:

1. Google initially ignored jpeg-xl but then everyone jumped on it and now they feel they have to create a post-hoc justification for not supporting it earlier which is tricky and now they have a sunk cost situation to keep ignoring it
2. Google today was burnt by the webp vulnerability which happened because there was only one decoder library and now they're waiting for more jpeg-xl libraries which have optimizations (which rules out reference implementations), good support (which rules out libraries by single authors), have proven battle-hardening (which will only happen over time) and are written safely to avoid another webp style vulnerability.

Google already wrote the wuffs language which is specifically designed to handle formats in a fast and safe way but it looks like it only has one dedicated maintainer which means it's still stuck on a bus factor of 1.

Honestly, Google or Microsoft should just make a team to work on a jpg-xl library in wuffs while adobe should make a team to work on a jpg-xl library in rust/zig.

That way everyone will be happy, we will have two solid implementations, and they'll both be made focussing on their own features/extensions first so we'll all have a choice among libraries for different needs (e.g. browser lib focusing on fast decode, creative suite lib for optimised encode).