New formats are not supported on all browsers so you wind up having to support the mainstream formats as well as one or more new formats. Rather than benefiting from reduced storage costs, your storage costs get multiplied.
Performance is also a problem, particularly when people add "yet another polyfill" to get an image to work without native support.
Once I get into eyeballing images closely I also end up questioning if the compression gains are real. For instance, one I read a comment on the article here
https://calendar.perfplanet.com/2018/is-avif-the-future-of-i...
and looked closely I couldn't unsee that the AV1 had very different artifacts than the other compressed images. Not necessarily worse, but definitely different. Unlike the poster I liked the way the suit rendered but definitely there was a lot of blocking around edges that the poster interprets as "pixelation on the flag".
Like many things, the costs of switching image formats are definite, but the benefits are questionable.