video decoding on a general-purpose cpu is difficult, so most devices that can play video include some sort of hardware video decoding chip. if you want your video to play well, you need to deliver it in a format that can be decoded by that chip, on all the devices that you want to serve.

so it takes a long time to transition to a new codec - new devices need to ship with support for your new codec, and then you have to wait until old devices get lifecycled out before you can fully drop support for old codecs.

Backwards compatibility. If you host a lot of compressed video content, you probably didn't store the uncompressed versions so any new encoding is a loss of fidelity. Even if you were willing to take that gamble, you have to wait until all your users are on a modern enough browser to use the new codec. Frankly, the winner that takes all is H.264 because it's already everywhere.

AV1 is still worse in practice than H.265 for high-fidelity (high bitrate) encoding. It's being improved, but even at high bitrates it has a tendency to blur.