Chrome frame takes about 30-45 sec to DL & install. Doesn't require a browser restart and doesn't require admin rights. Its incredibly slick. and then you're free to work with HTML5 and CSS3 in ie6+
So they're stuck with IE6, because that was the latest & greatest when large custom web-based applications really took off, so they're all certified for IE6 only and would have to be replaced at enormous cost before the users can be allowed to upgrade (or install chromeframe, for example).
It's a big barrier.