Here's a modern web development setup:
The machine on the left runs:
a web server, multiple databases, Eclipse IDE, Sublime Text 2, test suites, source code control, Chrome, Firefox, terminal sessions, Skype, IRC, Spotify..
The machine on the right runs:
..because IE6 just WILL NOT DIE, EVER.
And every time you release some web software (assuming you want it to be used within one of the benighted organisations that still use IE6 - local government, big-but-slow companies who are paralysed by fear and cost) you'll have to test every bit of every page all over again JUST ON IE6.
AND - the rest of us are all paying for it, in wasted development effort, longer development timescales, increased bug-fix cycles, and lost hair.