And slowness of cross-browser deployment. Both for stupid and very valid reasons.

As an example of the latter:

I think Opera Mini (which I ran the development of for its first decade) still has somewhere around 150-200 million monthly unique users, down from a peak of 250M. Pretty much all of those users would be quite happy to receive this image quality improvement for free, I think. (Assuming the incremental encoding CPU cost isn't prohibitive for the server farms.) Opera Mini was a "launch user" of webp (smartphone clients only) for this particular reason.

Many of those users devices are Nokia/Sony Ericsson/etc J2ME devices with no realistic way of ever getting system-level software updates. They are still running some circa 2004 libjpeg version to actually decode the images. It's still safe because the transcoding step in Opera Mini means that they aren't exposed to modern bitstream-level JPEG exploits from current web, but it underscores why any improvements targetting formats like JPEG is still quite useful.

Opera Mini for J2ME actually includes a very tiny (like 10k iirc) Java-based JPEG decoder since quite a few devices back then didn't support JPEG decoding inside the J2ME environment. It's better than having to use PNG for everything, but because it's typically like 5x-10x slower than the native/C version even in a great JVM of the time it really only makes sense to use as a fallback.)