Why doesn't the new yahoo beta mail work on internet explorer?

It's odd. I was on internet explorer the other day and it said there was a new Beta Mail for yahoo so i switched. It worked fine. Then the next day I went to open my mail and it said that my browser (IE) doesn't support the new Beta Mail? hmmmm?. So I had to switch to Firefox. I have Windows 7 and my IE is Explorer 9 Beta. Why wouldn't Beta Mail work with IE 9 Beta or Windows 7......I don't get it. Someone help, please =)