Google, (the Google Chrome team), have stated before that "[JPEG XL] doesn't provide significant benefits over existing image formats" [1] and have been vocal in there disinterest in shipping support for it (they deprecated experimental support) [1]
My guess is that the newest/latest JPEG encoder developed by Google researchers, Jpegli[2], has a lot to do with this. Jpegli has been described in a Reddit comment as "a JPEG encoder that was developed by the JXL [JPEG XL] folks and the libjxl psychovisual model" and described to have superior performance to WebP (lossy WebP) [3]. that whole reddit thread has comments relevant to this discussion, specifically about the tradeoffs of supporting extra formats in browsers.