it has the best lossy image compression (not counting extremely low bitrate images, where AVIF starts to win)
it can losslessly recompress JPEGs for a free 20% space savings - no image quality loss
it supports parallel decoding for extra speed
it supports progressive decoding (viewing a lower quality version of the image while it loads), unlike WebP/AVIF which just "pop up" when you've downloaded the whole thing
it supports lossless
it compresses lossless extremely well (notably unlike AVIF and PNG which fall on their face with lossless compression)
it supports animation (though AVIF is generally a better format for animation, because it's based on a proper video codec)
it supports HDR
it has a very strong resilience against generation loss (the classic "JPEG degradation" of resaving images)
it is royalty-free
it otherwise has roughly every image format feature we've ever thought of included in its spec
If JXL is not the next image format then we will never ever get rid of JPEG and PNG. There has never been a more obviously superior image format in history.