If Microsoft had introduced Google Chrome style browser updates (either at the beginning or as a default XP service pack) this wouldn't be a problem. But even without problems of enterprise lockin, the average person doesn't update software that appears to work OK very often. Some of them are probably still using Windows 2000 or ME, which doesn't even support newer versions of IE but is still fit for most simple purposes

A significant proportion of internet users don't know what a browser is, and if they see a website that advises them they need to download a piece of software to get it to work, they instantly distrust it. In all honesty it's probably better overall that people who don't know about browsers do ignore websites with buttons telling them they'll enjoy a better viewing experience if they download and install extra software.

Like people using the latest cutting-edge iPads, they'll just have to miss the occasionally useful flashiness that their default browser doesn't attempt to support. The average person that hasn't upgraded by now probably isn't going to be swayed by yet another broken rendering or "this site is only viewable in IE7+ or Chrome or Firefox"

The only party that can seriously reduce IE6 market share is Microsoft, by using OS updates to force upgrades and creating an "IE6 mode" that allows those kludgy enterprise apps to continue to function.