We have no control over whether or not our users use chrome frame. Asking them to install it isn't always an option, especially for government installations.
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.