Sound more like a chicken and egg issue, look like "big tech" are mostly using the format for archival purpose.

As Mozilla put it, adopting a new format increase the attack surface ( that is already quite large ) of a browser, they can not simply take the reference decoder and integrate it into their codebase, they also have to use resources toward maintaining it. Firefox currently integrate jxl support behind an experimental flag, but that could mean the feature is in testing and could be deprecated in order to better allocate their resources.