Quick answer: Because our alpha software isn’t going to support your beta browser.
Long answer: The IE7 team, while working hard to improve CSS support, is still missing in a few areas. Now, that normally wouldn’t be a problem (hey, we support IE6, don’t we?), but given the rendering codebase is changing so much between releases (see the IEBlog at http://blogs.msdn.com/ie/archive/2005/07/29/445242.aspx), we have made the decision to not support IE7 until after it reaches GA. With that said, please feel free to use your own CSS styles and design something for the current beta release of IE7 if you feel that you must use it right now. Otherwise, check out a real browser.