Speed and size - if you you were using images, you are going to incur an extra HTTP request, while using this method everything needed to draw the logo is already at the client-side. In terms of size, the HTML and CSS needed to draw the logo could well be smaller than that needed for an image. In this case the HTML+CSS is around 2.6K (without the Google analytics code), while a screenshot of the logo is around 10K - and the HTML+CSS version will scale to any size without pixilation.

Google's Marissa Mayer did a talk about Google's quest for speed a few years back, where she showed that an HTML version of the Checkout icon was faster than an image: http://www.bitcurrent.com/marissa-mayer-at-velocity09-and-go... (about halfway through that article).