Simply because IE6 still represents 27.21% of the web's population (or 15.21% depending on your numbers) as of July 2009.
Now I know some of you will probably tell me that if more and more sites stop supporting IE6, the browser will eventually disappear. That's a lie.
Picture this:
Corporation ACME has over 150 000 computers all running Windows 2000/XP. They also have a nice intranet site developed 7 years ago which works in IE6 quite well, but not so much in other browsers.
Do you really think they are going to invest money into fixing their intranet application when they control their complete IT infrastructure and who gets what updates? It's less costly to just postpone the update until they migrate to a new system.
A lot of corporations are in that situation.
Here is another example:
Business FooBar sells its products on the Internet. A little more than a quarter of their traffic is coming from IE6, which also means a quarter of their sales.
Do you think FooBar will simply block off those customers or annoy them with a huge notice telling them they are using a buggy browser? That would cost them nearly a quarter of their sales! As long as there is monetary value to supporting IE6 (and it does and will till its market share drops below about 8%), IE6 will prevail, which is also why Google won't be phasing out support for IE6 anytime soon.
Campaigns such as Browse Sad do not understand the mentality of the corporate culture (change is costly) and do not understand that in the end, consumers have a negligible impact on the worldwide IT ecosystem. The big corporations control it.
Consumers do have a growing impact but it is still insignificant compared to the impact corporations have.
And let's be truthful here: everyone who has the technical expertise and who could upgrade to a better browser already did. The rest are people still running outdated OSes, don't know how to upgrade, or don't have admin rights on their machine.