Layman's suspicion: adoption is hard when nearly everyone and their uncle knows and supports gif/jpg/png. At least for most end consumers there's no major advantage to adopting early. And in such a scenario most people adopt when they are forced to because everyone else adopted. So it's a hen-egg problem.

Ideally when you introduce a new format you support both the old and new format concurrently over a long time to allow for a gradual transition. The major advantage of webp/avif is that they need less storage space for the same quality. However if you have to store everything in an extra format whilst also keeping the old ones you are completely reversing that storage advantage and now need even more storage volume than before.

As far as I can tell AVIF has much better prospects of being the future image format anyways. In the long run that is. Plus it's open source and not just a single tech giant behind it. Suffers from the same slow adoption rates though.