The images arbitrarily expire after about two and a half months, and then (last time I used them at least), there's an interval of time where they don't put up new ones. I don't have time to download 11 GB of disk images every three months, and I certainly can't have MS arbitrarily making it so that I can't test in their awful browser for months at a time.

If you want to support IE, owning a copy of Windows is not optional.