Maybe, in the long run Google just wants to see what patent trolls may be out there waiting for either VP8 or Theora. In the short run, as Jason Perlow suggests, perhaps Google wants to cut down on the infrastructure costs of supporting half-a-dozen plus different video code variants. After all, if someone really wants H.264 video, depending on its container, they can view it with Adobe Flash, Apple QuickTime, or some other browser add-on program that includes H.264 support.
This seems to me to be the most likely explanation. There seems to be this illusion that HTML 5, the latest proposed version of the Web's foundation markup language, would somehow magically get rid of all the fights about video codecs and we'd all be able to view all video from any browser. What nonsense!
Sure, Ian Hickson, one of the editors of the HTML 5 standard, said, "One of HTML 5's goals is to move the Web away from proprietary technologies such as Flash, Silverlight, and JavaFX, but HTML5's codec-neutral video tag doesn't spell out which codec would be the official standard codec. In short, the World Wide Web Consortium's (W3C) ended up just pushing the same old Web video standard fights into a new arena. Now, besides the usual fights over which is better or more open, it's also about which one is better for HTML 5.
While I think the answer is the more open standards, VP8 and Theroa, what I think doesn't matter much. This is just a round in another standard fight: the real question is who will win and get control and money. I doubt we'll see a winner this decade. The W3C moves very slowly and these are multi-billion dollar companies struggling with each other.
All the browser companies have their favorites. Google has just declared which ones they like the most. For users, it won't make a damn bit of difference. In 2020, developers are still going to fighting over video standards, there still won't be an official HTML5 video codec; and no matter what browser you'll be using, you'll still need one or more helper applications to be sure that you'll be able to watch all 800,000 plus funny cat videos.