The specification was frozen "only" in December 2020. It is enough time for a lot of software to add support to it (Gimp, ImageMagick, Krita, FFMpeg, Adobe Camera Raw). But it will take a long time before camera start using it. And since a lot of popular graphic software cannot export to JXL yet (photoshop, clip studio, lightroom, ...) most professional just can't use JXL (short of saving a lossless image and then converting it to JXL in another software). And in general, since the format is pretty new, most people don't know about it. That is why being able to use it for the biggest browser would have been huge for speeding its adoption.