On the one hand, it's for charity.
On the other hand, it's a well known fact that Microsoft is losing browser market share, and that their browsers often lack the features of their competitors. I might be wrong, but I'm pretty sure that other browsers will support HTML 5 way before Microsoft does.
I'm venting a lot of IE6 frustration here, but I think Microsoft has been holding back the progress of the Internet for the last several years. Standards could have evolved so much faster...
EDIT: I'm a little cynical. And for those people who will vote me down and go on about how it's for charity: Microsoft is losing browser market share, and Google is rolling out a browser-based OS. Do the math.
I don't think it's about the marketshare, they are targetting their own users. It's probably more about getting rid of the dinosaur that is IE6 and moving forward (about time).