Here’s an interesting theory.
Although opinions vary on the reliability of browser market share figures, a quick scan through all the data reveals one interesting commonality across all the data sources: IE6 market share peaked at around 95 percent sometime in mid-2004. If 95% of the world is browsing with IE 6, pursuing browser independence is a waste of time. If you don’t have to worry about browser independence, you are suddenly free to exploit advanced browser techniques like XMLHttpRequest.
The super-saturation and monoculture of IE6 from 2002 to 2004 created an incredibly rich, vibrant development platform where developers were free to push the capabilities of the browser to its limits. Without worrying about backward compatibility. Without writing thousands of if..else statements to accommodate a half-dozen alternative browsers.