There are actually quite a few organizations that have internal solutions developed for IE6, and since IE6 deviated a lot from standards, they can't upgrade to later more standard-compliant IE versions, since the solutions would stop working correctly then. So the organizations have policies that employees must continue to use IE6 and aren't allowed to use different browsers or versions.
Since Windows XP came with IE6 and many still have that OS, it also means that a lot of less computer-savvy users still have IE6.
As for myself, I can't upgrade to a later IE version than 6, since I have another program that will stop working then (yes, it's old software but one I'm using). I can however use Firefox instead, and do!