Dwight Silverman, tech writer/blogger at the Houston Chronicle, explains it best:
Now, IE6 wasn’t a particularly great browser when it launched in 2001, and at this point it’s a positively awful one. It doesn’t fully support many of the best ways Web designers build sites and browser-based applications, and it’s prone to security issues. It doesn’t, for example, offer any protection against phishing scams, in which users are tricked into going to a malicious Web site.
Web developers would love to quit supporting it on their sites,. But that 17 percent usage rate is a problem. They have to spend time and resources tweaking their sites so they work with IE6, just because so many people are still using it…
Web developers are starting to rebel, opting to no longer ensure that IE6 will work with their sites. And it’s not just small, out-of-the-way corners of the Web — YouTube recently began warning IE6 users that they need to upgrade if they want to continue using that wildly popular video-sharing site in a glitch-free manner.
I’m totally on board that bandwagon.