A post on the IEBlog touched on this topic recently and basically concludes that dropping IE6 support is simply not an option, for a variety of reasons. One of these includes the substantial user base that clings to the old browser, which still represents millions of web-connected users. Another and arguably more important reason is the commitment level Microsoft has to maintain.
They are stuck with the fact Windows XP shipped stock with IE6, and they have an obligation to continue supporting it for the lifetime of the product. That's a problem for Microsoft since extended support for Windows XP – and thereby IE6 – continues for another five years.
We're talking just Microsoft here, of course. Any other company around the world can choose to support whomever they wish – YouTube will soon drop support for IE6, most web developers cater exclusively to IE7 or higher and many other sites are urging users to upgrade as well. Ultimately, IE6 will die, but it is clear that Redmond expects that death to be slow and painful.