Free software. The Cisco H.264 codec supplied with Firefox cannot be distributed with other software, despite being "open". From https://www.openh264.org/ , there seem to be licensing costs involved with the usage of this code, which have been waived for some uses, implying they were not waived for others:

> We will not pass on our MPEG-LA licensing costs for this module, and based on the current licensing environment, this will effectively make H.264 free for use on supported platforms.

EDIT: To clarify, such licensing requirements on H.264 may make codecs fall outside of the guidelines of software distributions allowing only free software, making them supply their browser without one.

Many RTC stacks (and specifically openh264) also only support H.264 baseline profile. So you are likely to get higher quality with VP8 (subject to variations in the quality of the implementations on each endpoint, of course).

Some services use WebRTC in a "hub and spoke" model rather than true peer-to-peer, and they often require VP8 due to a desire to support low-end Android devices that don't have hardware H.264 decoding.