The boring answer: because it's going to form the underpinnings of the entire internet within a few years, and most major browsers (including Chrome, now) support it. The better answer: because it's really cool, enabling webtoys like this, without plugins.
A year ago, this cool (as in smoooooth) visualization, which plays music, displays fluid animations, and integrates live with Twitter, would've been something you'd have to make in Flash or Silverlight, requiring programmers to learn a new languages, and viewers to download plugins. Now it's possible to pull off using just the new HTML5 spec, which includes support for video and audio embedding, Canvas�for scriptable animations�and all kinds of other rich content that'll make current HTML-based webpages look like Geocities templates. Did I mention that YouTube could lose Flash altogether? Yeah, that.
Actually, the 9Elements design studio seems to have written their entire homepage in HTML5, so grab the latest build of Firefox, Opera or Chrome and have a look at what it can do. You'll be seeing a lot more of this before too long, and if your newest gadget's software supports it, consider it a worthwhile feature.
