I think libjxl's development is stalled because the format is hard to work with. It wasn't super hard to drive meaningful improvements to AVIF.

Yes, SVT-AV1 received and continues to receive development efforts from devs at big companies, but the number of core contributors has always been somewhat small. Definitely more resources, but the entirety of the original AVIF work was done by two people.

I'm able to utilize my experiences generally in image coding to work on my encoders. This should translate to JPEG XL, but I feel held back by how algorithmically complex compelling implementations of the coding tools would be, and how to make those implementations fast. I think if the JPEG XL spec was incredibly intuitive, community contributions would have gotten it a lot further. Heck, my own efforts may have gone to it instead of SVT-AV1-PSY's AVIF encoding.