Maybe because it's resized down. In contrast to what others have replied, I personally find PNG file sizes to be horrible for thumbnails - even after running through optipng.
File size I imagine. Typically, a PNG will be 4 or 5x the size of a low/mid quality jpg. 31kb vs 150kb when you're delivering a bazillion pages makes a difference if you don't have the bandwidth.
Granted, that seems quaint nowadays, but maybe MS doesn't give two shits about making sure the blogs have the bandwidth and resources they need for nicer looking graphics.
Actually, for pictures of text, a PNG is usually 1/2 or 1/3 the size of a jpeg. Try taking a screenshot of a hacker new's comment section and saving it as PNG or jpeg and check the results.
http://blogs.msdn.com/cfs-filesystemfile.ashx/__key/communit...
I see this problem in almost all MSDN blogs, must be part of a convoluted CMS workflow or something.