Well in fairness when WebP support was added in Chrome over 10 years ago it was a massive, massive improvement over the existing image formats that were being commonly used on the web.
JPEG XL's problem is that WebP has now existed for 10 years and has widespread support.
I don't think that's the entire story. Google actively pushed WebP, for example recommending it to webmasters in their page speed evaluation tools (and site performance, as judged by Google, is a factor in your search ranking).
If they invested the same resources into improving and promoting JPEG XL, we'd be using JPEG XL. I'm not saying the outcome is objectively worse, but ultimately, they did pick the winner here.
If you look at slide #14, you'll see Opera was an early adopter.
Firefox published a "No" blog-post in 2013, and Safari removed WebP support from Sierra preview in 2016, eventually adding it back in 2020. Stuff happened.
And yes, when WebP was created there was a real, non-incremental, need for a Web-oriented image format. Nowadays, it's just incremental improvement on this idea for browsers.
Mozilla didn't support WebP for years. I'm fairly certain one of the official reasons they gave for not doing it was that it wasn't a massive jump in quality over JPEG, which obviously would have been good for the web if it was true and motivation for them to support it sooner than they did.
Notably, when they did add it, Edge did so around the same time, which seems to me suggests some politics in the background, (not in a conspiracy sense, just an agreement based on interperability) which I suspect might apply this time too.
Worth noting that JPEG XL has a pretty good lossless mode too, however, I think in many cases you actually want to just use a lossy AVIF or JPEG XL in places you'd previously have used "lossless".
It was kind of an artificial split, much like text, alpha transparency or line drawing might be done as PNG, not because you wanted them to be "lossless" but because JPEG would mess them up due to a lack of tools for reproducing non-natural photo content.
basically: JPEG XL does things right, WebP lossless is also not bad (14.8 % worse than JPEG XL), PNG is ok for its age (46.3 % worse), AVIF didn't focus on lossless (63.0 % worse)
JPEG XL's problem is that WebP has now existed for 10 years and has widespread support.