I don't know about others but I find much less old version of Firefox, safari and chrome than IE 6 users access my sites. It seems (this is only a personal observation) that users of these alternative browsers are more likely to update frequently.
I think you're right. Another big factor is also the automatic updates of Chrome and Firefox. IE updates have to go through the windows updates and people tend to ignore those as much as possible.
Another factor about IE6 is that it's only used in organizations and companies where people can't install their own programs, so they don't bother calling up IT (sometimes outsourced IT which is expensive) just to install a browser.
