When IE6 stagnated, it had a market share of over 95%. So if a web developer wanted to take advantage of new HTML/CSS features, only 5% of their users would not

When IE6 stagnated, it had a market share of over 95%. So if a web developer wanted to take advantage of new HTML/CSS features, only 5% of their users would notice, and so it wasn't really worth the cost. This meant that all websites had to work in IE6, so there was less incentive to switch browsers. Now IE market share is closer to 40%, so whilst they're still the market leader, they no longer have a majority. Even if Microsoft ceased development on IE9 now, web developers could still use new HTML/CSS features and reach 60% of their audience. This would mean more websites using new features, which would result in a faster switch from IE. If you have a monopoly, you can afford to sit back and allow technology to stagnate. If you don't, and your competitors are constantly developing new features, you'll lose market share rapidly unless you keep up.