The implementation is what I was thinking of. I've also heard claims that VP9 is inherently slower to encode than H.264, but no idea if that's accurate. AVC/H.264 has very broad hardware support. For example, the 2019 MBP I'm using right now can't do hardware-accelerated VP9 encoding, but even 2011-ish MBPs can do H.264 acceleration in both directions. Intel's support matrix: https://en.wikipedia.org/wiki/Intel_Quick_Sync_Video

For PCs with dedicated GPUs, Nvidia's matrix doesn't even mention VP8/9 encoding, and for decode there are a lot of VP8/9 "no"s for GPUs that have "yes" for H.264: https://developer.nvidia.com/video-encode-and-decode-gpu-sup...

AV1 looks like it's getting broader support, but it's still new. Zoom's release notes mention they'll use AV1 if the participants support it, and I don't see a similar note about VP8/9.