The fact that JXL is a new convenient crusade for people that hate on Chrome. You'll notice that noone is screaming at Mozilla for the same choice.
However Mozilla has not completely dropped the feature like Google.
Chromium's JPEG XL feature request is marked wontfix but still possible to comment or express interest (star the bug).
Chrome team themselves quoted no interest of those browsers as the reason for removal of the flagged code. Seems like an obvious way to pressure Google into supporing JXL would be to get Mozilla to launch their support, together with Edge and Safari?