Let's face it I know lot's of people still using XP and even my Father in law using Vista. Never has thought about using anything but IE. I finally got him trained on keeping things updated.
This is the norm for a lot of users in the World. Not everyone is a geek or thinks as much about security or updates as probably most of us do. For that matter even know about these zero day exploits that could compromise their PC. I think the Apple fiasco shows how 600,000 computers can easily be infected and stay infected even after the press has gotten the word out. Some are like Apple users who feel they are immune or PC users who overly trust their Anti Virus security suite to stop everything.