[Retric]

JPEG-XL is optimized for the low to zero levels of compression which isn’t as commonly used on the web, but definitely fills a need.

Google citied insufficient improvements which is a rather ambiguous statement. Mozilla seems more concerned with the attack surface.

[formerly_proven]

JPEG XL seems optimally suited for media and archival purposes and of course this is something you’d want to mostly do through webapps nowadays. Even relatively basic uses cases like Wiki Commons is basically stuck on JPEG for these purposes.

For the same reason it would be good if a future revision of PDF/A would include JPEG XL, since it doesn't really have any decent codecs for low-loss (but not losless) compression (e.g. JPEG sucks at color schematics/drawings and losless is impractically big for them). It did get JP2 but support for that is quite uncommon.