1. We need a video codec that everybody agrees on for WebRTC.
2. We need a way to play H.264 on Windows XP and other operating systems that don't provide native H.264 playback, for compatibility with almost all HTML5 <video> content on the web.
If it weren't for problem #2, then I would agree with you more. But, really what Mozilla seems to have said is "Hey, if you provide a free H.264 decoder without field-of-use restrictions, then we'll compromise on WebRTC so we can make sure everybody on every platform can play all the HTML5 video content available on the web."
I don't have an opinion on VP9. But, there's no VP9 content on the web now. There's tons of H.264 content on the web. Mozilla got burned waiting for other players to help with WebM adoption before. Waiting for VP9 would probably just end up with us getting burned again.
FWIW, I work at Mozilla but not on this stuff, and I learned all I know on this topic from Brendan Eich's blog post.