I tried to find out the reason, and it is not clear why this happened, some people think it's a patent thing, although that has been disputed. It's unlikely that it is a technical reason. It seems more likely to be political. The googs has more control over the development of webp than jpegxl, so they push that.
But if you want a systemic analysis the googs can only do that because they dominate the browserspace.
If Firefox gained more popularity while supporting jpegxl, the googs would have to follow suit and support it too.
I don't know if that this is necessarily the death of the format. If the Mozilla foundation is clever they can accumulate support for those useful features that the googs is not implementing. While nobody will switch browsers or change their web-development pattern for this one format, it is likely that a big enough number of feature differentiations can make people change browsers again.