HTML 5 Video spec isn't even finalized
I think that the fact that Google only enabled h.264 HTML5 video on youtube has more to do with the fact that all their videos were already encoded in that format (at 3 different resolutions), for iPhone and Android support. Therefore, it was relatively easy to just turn on the switch for beta HTML5 embedding.
Transcoding all those videos to Ogg Theora (with multiple copies for SD, HQ and HD) would require a major computing effort and storage space availability, that, sadly, just isn’t worth it at this point. Just transcoding videos from the FLV format to H.264 took MONTHS in 2007, and that was before 720p and 1080p HD content.
Also, Apple should be blamed for being the only browser vendor refusing to feature Theora support for its video tag support.