The problem with trying to replace JPEG is that for most people it's "good enough". We already had "JPEG 2000", which would have been a step up in terms of performance, but it never saw any real adoption. Meanwhile, "JPEG XL" is at best an incremental improvement over "JPEG 2000" from the user's POV, which raises the question why people would care about this if they didn't about the previous one.
JPEG2000 ran into a patent license trap. JPEG XL is explicitly royalty free.
But that doesn't address the point that JPEG XL is only marginally better and has a gigantic mountain to climb if it ever hopes to displace JPEG (and likely never will given the vast set of JPEG files that exist and will never be converted).
JPEG XL can losslessy transcode JPEG into a smaller format. JPEG2000 (or WebP or anything but Lepton[0]) didn't offer that. Besides, we have gif and png for approximately the same space. gif still isn't gone. Displacement isn't necessary for a new format to become useful.