In a lot of cases (including mine), IT operators are working with zero budget. While there's the obvious argument that upgrading from a free browser to another free browser doesn't cost any money, it does cost time; I work at a federal agency in which their web applications HAVE to work, and unfortunately, many of them were written for IE6. Rewriting those applications to support a new browser isn't free either.
In addition that that, there is time spent securing the installation. In the case of IE it's a little easier as I can manage it with policies, but it is absolutely critical that we're following DOD stigs regarding security, and setting up a central policy isn't free.
Regarding the PC comment, the laptop I replaced a month ago was a 1.0Ghz Pentium with 512M RAM. This was the upgraded laptop at the time, that I needed for java development. Compiling code with 512M on XP was adventurous. For the last 8 months I had it, the internal LCD had died, most of the keys didn't work, the WiFi radio was completely dead, and the power button would only work if I wiggled the frame and monitor. I made sure to use it on a plugged-in (ethernet network) to bypass the WiFi, plugged into an external monitor to get around the LCD, grabbed an external keyboard to type on, and wiggled it every day to power it on.
Before deeming it dead, they replaced the motherboard 3 times and the hard drive twice. I kept using the decrepit old machine because frankly, it was the nicest laptop anyone had on the team. They're refreshing the machines now, but we've only had IE7 in the environment for about a month. I am lucky in that I can use Firefox, but only within the firewall, and not at all to browse any external sites.
Sorry, I kind of got off on a tangent here, but it's perfectly reasonable to understand that there ARE in fact reasons why it takes a long time to get off of IE6.