Some big companies simply do not want to take the "pain" of upgrades. I'm working as a consultant now, we are developing a project for a big international company on Microsoft CRM, that specifically works only with IE.
IE6 causes a lot of problems even with Microsoft technology. Nonetheless this client do not want to upgrade their browsers. This is not the first time this happens.
There are many office jobs out there that don't allow their employees to install software on their computers or give them anything more than what comes with a default XP install, thus leaving them stuck with IE6.
