If IE6 has the UI that people accept, and if IE6 has the performance/resource utilization that those people like (compared to modern browers, it is pretty lightweight except on javascript-laden pages), and if IE6 already works for every site that they use, why should they switch? Why should they buy a new computer or more RAM just to run a new browser when their current one works just fine? Why should they download a third-party browser and then have to deal with even more updating and migrating their bookmarks and learning a new UI?
We need to understand that there are going to be a significant number of people running IE6 on Windows XP even a couple of years from now. Nobody has created a compelling reason for them to upgrade and it doesn't seem like it will happen until Microsoft stops issueing security upgrades for those products. Even then, a lot of people won't care if they get security upgrades or not. (I know of a small business that is using a ten-year-old desktop running Windows 98 as their only file server today.)
Basically the IE6 user base is upgrade averse as it gets, and a new major revision never cures that. IE6 can only be eradicated by a coordinated desupport effort, and that's a total prisoner's dilemma mess. No one's going to risk their traffic for it.