A post from Internet Explorer Team's blog explain why Microsoft still support IE 6:
The engineering point of view on IE6 starts as an operating systems supplier. Dropping support for IE6 is not an option because we committed to supporting the IE included with Windows for the lifespan of the product. We keep our commitments. Many people expect what they originally got with their operating system to keep working whatever release cadence particular subsystems have.
As engineers, we want people to upgrade to the latest version. We make it as easy as possible for them to upgrade. Ultimately, the choice to upgrade belongs to the person responsible for the PC.