Think about this: if you have a legit copy of Windows XP you're HARASSED to upgrade to the latest version. If you have an illegal copy, you're either smart enough to ignore the harassment, or you constantly fail the required product validation before upgrading.

I think this proliferation of IE6 is because it was the last upgrade that didn't require validation. It lives on through piracy, which also promotes insecure computers that don't have the latest updates.