Because technical superiority isn't the only factor? Obviously, you can use anything you want. As can Google, and they've obviously decided not to support H.264 any more, at least in their browser.
The obvious reason for moving away from H.264 is financial cost. Somebody has to pay for the H.264 patent licences, and Google can't rely on the user having Quicktime installed. Users such as, for instance, a Linux user. Or an Android or Chrome OS user. So Google foots the cost of the patent licence. Obviously by supporting H.264 in their software, they are encouraging the proliferation of a patent encumbered algorithm. And what if MPEG-LA decides to change the licencing conditions or substantially increases the cost of licencing at some time in the future? And of course they need to pay the licence fee because of the threat of litigation, and it'll be a lot more painful to remove support when H.264 is de-facto standard for HTML5 video. Software patents are enforceable in the US, and bear in mind that the H.264 patent holders include Microsoft and Apple, who don't exactly have many reasons to go lightly on Google. Even if they win the suit, it can be dragged out for many years and cost them potentially hundreds of millions.
There is also an ideological aspect to it, and just general common sense. Why support a patent-encumbered format when there is an almost as good open source, patent-free format owned by them?