As I said earlier and have repeated:

I follow JXL relatively closely and I still am not 100% sure why they went through with this

If you've got a better guess please share. No one knows why they've done it except Google. The popular theory is that they're doing so to push WebP+AVIF instead, because it's one of few ideas that makes sense. We know their decision is political in some nature:

• they intentionally misrepresented the interest from companies and the community on their bugtracker
• they gauged this "interest" after only about half a year of being hidden behind an opt-in flag, which is not a fair assessment as websites could not activate JXL delivery
• the public benchmark that they published was conducted so poorly it's hard to believe that it wasn't done intentionally
• after a thorough rebuttal to the flawed methodology was posted, Google has not responded, redone their benchmarks, or reconsidered the data
• benchmark after benchmark shows JXL dominating AVIF by a similar margin that AVIF dominates WebP, along with the large featureset that JXL carries compared to AVIF and especially compared to WebP - yet Google claims that there's no clear benefit to the format
• AVIF and WebP were not subjected to this much scrutiny when being activated in Chromium. Those passed into live builds without much interest, and in the case of WebP there wasn't even a clear benefit over JPEG.

Making one or two of these mistakes before correcting them might be understandable, but making all of them and going radio silent when called out for them means they're doing this with a motive that is not data-driven or in good faith.