Quite frankly that is because IE is a browser that we have to live with users using. If it was a bit more realistic I wouldn't support IE at all (no matter the version), I would even take steps to break my site in IE since IE is the last browser which has any meaningful marketshare and which is not automatically updated to the latest version.

So if you use IE when you are not forced to, you drag the rest of us down. And that frankly isn't cool.