Because Flash has 99%+ marketshare, whereas the H.264 video tag has <1% marketshare, so Mozilla are in a position to influence the latter but not the prior.
Most of the video delivered on the web today is in h.264, using Flash as a way of delivering it. This is why it's trivial for YouTube and Vimeo (and everybody else that currently uses Flash) to use h.264 in <video> -- all of their video is already in h.264.
