IE6 doesn't support HTML 4 properly and it especially doesn't support CSS 2.1 properly. The effect is that whereas normally the cost to support a given browser is relatively low (especially more standards compliant browsers such as those based on webkit) the cost to support IE6 is very high, especially if you demand that the appearance on IE6 should be treated with equal importance as other browsers. The result is that even though IE6 usage is maybe 20% of internet users as a whole (and perhaps less on a given website), supporting those users is vastly more expensive than supporting other users.