It's because IE6 was the last IE release before the end of the first browser war. Previously, Microsoft had been releasing browsers every year, but they stopped

It's because IE6 was the last IE release before the end of the first browser war. Previously, Microsoft had been releasing browsers every year, but they stopped after Netscape died. It wasn't until Firefox started to gain market share, almost 5 years later, that IE7 came out. In the intervening years, IE6 had the chance to gain about 99% of the market. (Not sure if all my figures are accurate, but that's the gist of things).