Oh, the reason’s actually really simple: Because Microsoft install it with Windows. I’d love to get usage statistics from the EU, but I can imagine that usage of IE would go down exponentially when you open up the choice for the uninformed. The people who just buy computers and don’t take an interest in the other options for anything, especially internet browsing, are never exposed to the variety of choice.
This, combined with the fact that people, especially in relation to computers, always have the “if it ain’t broke don’t fix it” mindset, so the more that webmasters make allowances for IE the more we’re encouraging people. There’s nothing intrinsically wrong with IE; its just that its absolutely foul when compared to the other browsers. That means that if you never compare it to anything, you never get a perspective on just how ludicrously retarded it is. As such, people never really understand how unbelievably full of fail it is until someone shoves a new browser under their nose, which, at the moment, only the EU is doing.
Case in point, I have a friend who once told me that his hard drive kept getting full. It was 4GB. It wasn’t until two years ago that he finally decided to convince his parents to replace it, almost eight years since it had been initially purchased. He had to shut everything down just to open Sim City 4, which played like slow piece of crap when he tried to show me anything. I brought him over to my house and showed him by uber computer. He was actually surprised at how slow his was. My dad did the same thing; on buying his new computer, he honestly asked me how I could have let him continue using his old one for so long.
Conclusion: Lack of information is a bitch. If people knew how lousy IE was, they’d switch in a heartbeat. Unless, of course, they’re the UK Government.