Well, lots of large companies who use Web-based systems often use older browsers as opposed to the latest ones. For example I know for Fact that Vodafone use IE5 in all of their stores as the Web based system they use works perfectly well and is stable. In cases like this, why bother upgrading when it works fine as it is? 😛
Because phpBB3 is packed full of extra features and eventually support for phpBB2 would stop (which by the way now has...some time ago actually). Vodafone don't care about fancy features or an improved Interface, as long as the system does the job, does it well and doesn't cause any problems...why invite a host of new problems by unnecessarily upgrading browser, and consequently having to upgrade the system...which in turn costs a lot of money? :great:
Murder hit the nail on the head for the most part, however that's not the full story. If your website is getting a decent amount of IE6 traffic, it probably isn't a store, but rather someone with Microsoft Auto-Updates turned off that doesn't know any better. People like this would be people who just arn't great with computers.
For example my mom still uses IE7 a lot on Vista, though she's starting to use Opera 10 more and more.
Because things change and advance. If they didn't then where would technology be today? I thought the same thing with the PS1 then came the PS2. Now its the PS3 that's at top.
Otherwise people would lose interest in the product honestly.