There's a lot of people out there who simply never upgrade anything. You'll be lucky if they even install patches. It may be laziness, stubbornness, lack of knowledge, or any of a whole load of other possible reasons.
I was at a hotel a few weeks ago, and they had a "free internet booth". It consisted of an old and very dirty PC running Win98 and IE6. I doubt the staff had even looked at it in years, beyond rebooting it every now and then. It worked, and it was good enough for a service they were offering for free, and they didn't really care beyond that. That machine will still be running IE6 until it breaks down permanently.
My parents' use Firefox, but only because I installed it for them. Their XP machine came with IE6, and they certainly wouldn't have upgraded the browser without my assistance (they wouldn't even have known that it could be upgraded). Many others don't have someone to hand with technical knowledge. Again, they'll all probably be running IE6 until their machines need to be replaced.
But the really big holdouts are corporates. There are companies out there which spent a lot of money developing custom browser based internal systems ten years ago or so, when it started becoming a fashionable way to write software. Sadly, virtually all the browser-based apps written back then were written for IE only. Many of them used custom ActiveX controls to make up for the lack of UI functionality in the browser. A lot of these systems will not work in anything other an IE6, and most of the developers who wrote them have long since moved on. A lot of companies that invested in them are very reluctant to give them up.
But the good news is that despite all this, IE6 usage is dropping off quickly now.
Many developers (ourselves included) have officially dropped support for IE6 from our sites. Many others are happy to support it enough for their site to work, but not trying to make it look good in IE6.