HEIC faces a few struggles with adoption:

1. It's patent encumbered.
2. It's not free as in "free beer", someone has to pay for it.
3. It's not free as in "free speech" - you can't use, modify, and distribute software or formats without restrictions due issue #1.

AV1 and AVIF don't have any of these issues –someone correct me if I'm wrong or missing something– so anyone is able to include/distribute the software and modify to their needs.