From the browser makers' point of view there's quite a bit of risk with introducing a new image format. libjxl is written in C++ so undoubtedly will be full of undiscovered security issues. I'm sure that someone will write a decoder in a safer language, but that work still needs to be done and/or finished, and then integrated with the browser. At the same time there are to 5 significant places probably 0% of websites that host .jxl files. So at the start it's all downside and almost no upside.
(Chicken and egg problem here of course which is no one will create the websites until there is wide browser support.)
(Chicken and egg problem here of course which is no one will create the websites until there is wide browser support.)