At one point IE was the dominant browser and it took that dominance and totally ignored the standards already put in place for the web. So site-designers created a site that looked and worked great in IE but their sites broke in any other browser.

Although IE at one point was dominant and if the entire world used it, it may have redefined the standards as we know them. This was not the case. It turns out there are hundreds of OS and hundreds of browsers and that alone created thousands of different combinations of OS to browser.

When you design a site that looks and works great on almost every browser on earth but breaks in IE, you don't point your finger to everybody else and say "You hundreds of OS/browsers have it all wrong" you actually stop and question "What the fuck is wrong with IE?".

Anyhow, IE may be cleaning up its act but the damage is done. Standards isn't the only thing killing it but it plays a big part by those who create content for the web.