The reasons aren't mysterious; it may be worth going over here:
1. Windows XP. XP installation media only ever came with IE6, never anything newer. Given the frequency that most corporate workstations get reassigned and wiped and reimaged, IE6 comes out of the reinstallation casket all the time.
2. Intranet systems. For most corporations of size, internal browser-based applications get the company's business done on a daily basis. Optimizing for this case is more important than optimizing for public web browsing. And these apps were written by the lowest-available-cost developers, many in outsourced shops that are long gone, who barely understood anything about the underlying technology and inadvertently rely on all sorts of browser misbehavior and bugs. (Also, the real limitation here is Microsoft-imposed, that you can't easily run multiple versions of IE on the same system, because so much of what we call IE is really integrated operating system components. IE6 for intranet alongside IE8/9 for the public web would be an optimal case but is not practical.)
3. Other browsers do exist. In several large corps I've seen, the policy (unofficial or official) is to use IE6 for the internal apps, and go ahead and install Firefox for public web browsing. The users savvy enough to install Firefox will do so; the users that don't know what a browser version is aren't missing anything.
4. User training. These are the users that will call the helpdesk and complain when their intranet app opens its popup in a tab instead of a new window and they can't find it because the instructions on their Post-it say to click on the window in the taskbar. No IT department wants to deal with that.
5. Cost-benefit, which is really a superset of everything above. There's still no compelling business benefit for most companies to buy Windows 7, so they're still reinstalling XP with IE6 all the time. There's no profitable business plan to be shown for spending time and money upgrading internal apps to work with IE8/9 and retraining users to the new workflow. Remember that these corporations are always in existence to make money. They aren't there to serve some Platonic ideal of everybody using the World Wide Web with the newest technology.
1. Windows XP. XP installation media only ever came with IE6, never anything newer. Given the frequency that most corporate workstations get reassigned and wiped and reimaged, IE6 comes out of the reinstallation casket all the time.
2. Intranet systems. For most corporations of size, internal browser-based applications get the company's business done on a daily basis. Optimizing for this case is more important than optimizing for public web browsing. And these apps were written by the lowest-available-cost developers, many in outsourced shops that are long gone, who barely understood anything about the underlying technology and inadvertently rely on all sorts of browser misbehavior and bugs. (Also, the real limitation here is Microsoft-imposed, that you can't easily run multiple versions of IE on the same system, because so much of what we call IE is really integrated operating system components. IE6 for intranet alongside IE8/9 for the public web would be an optimal case but is not practical.)
3. Other browsers do exist. In several large corps I've seen, the policy (unofficial or official) is to use IE6 for the internal apps, and go ahead and install Firefox for public web browsing. The users savvy enough to install Firefox will do so; the users that don't know what a browser version is aren't missing anything.
4. User training. These are the users that will call the helpdesk and complain when their intranet app opens its popup in a tab instead of a new window and they can't find it because the instructions on their Post-it say to click on the window in the taskbar. No IT department wants to deal with that.
5. Cost-benefit, which is really a superset of everything above. There's still no compelling business benefit for most companies to buy Windows 7, so they're still reinstalling XP with IE6 all the time. There's no profitable business plan to be shown for spending time and money upgrading internal apps to work with IE8/9 and retraining users to the new workflow. Remember that these corporations are always in existence to make money. They aren't there to serve some Platonic ideal of everybody using the World Wide Web with the newest technology.