Way wider browser adoption, potential to evolve together with AV#, since it's using a container format, so it shouldn't be limited to AV1 base. I.e. sites just need to adopt AVIF, and I expect then seamless ability to start using AV2 (and on) there without sites needing another wave of adding a new mime type and etc. which seems to be a huge hurdle.
It doesn't matter that AVIF uses the same container for AV1 or AV2 based encoding, if the browsers don't have the right decoder for it then they can't decode it.
An example of this is MP4: Browsers can decode videos encoded with H264 in MP4 containers, but not H265 even if it uses the same container, because one thing is the container and another thing is the codec, they're related but they aren't the same.
Notably, AVIF uses the HEIF container like HEIC. HEIF is an extension of ISOBMFF, mp4 files are another example of an ISOBMFF format. I'm surprised how ubiquitous that container format is becoming; webm uses the matroska / mkv format but I bet if it was created today they would have likely used something ISOBMFF derived
Browser adoption happens way faster than sites adoption (as current AVIF itself clearly demonstrates), so same container does matter to reduce contention on sites adoption side.
I.e. once browser adoption happens you'll be able to use AV2 for AVIF without the likes of Wikipedia taking another decade after that to add an additional mime type to their supported images.
