Several reasons, but the main concerns are that the software is 10 years old, it does not support key web standards, and it prevents us from presenting rich, interactive applications including video and voice.
Age: Microsoft's IE version 6 was released in 2001, is now fully 10 years old, and has been superseded by Microsoft by IE7 (released in 2006), IE8 (released in 2009), and now IE9 (released in 2011). This would be akin to using a computer today with Windows 2000, while that version has been supplanted by windows versions XP, Vista, and now 7.
Web Standards: IE6 is less compatible with — and lacking in support of — the latest web standards as published by the World Wide Web consortium for both device interoperability (eg, "will it work on this computer and that mac and this phone, etc.") and accessibility (terms defined through Section 508 of the US Rehabilitation Act). TTAP can do a better job delivering more content to more people using more kinds of hardware if we can save the overhead of making new websites reverse-compatible with IE6.
Interactive Applications: The web has evolved in the last ten years, from simple text pages to rich, interactive applications including video and voice. Unfortunately, very old browsers cannot run many of these new features effectively. Google Docs and Google Sites and YouTube have ended support of IE6 as of March 2010.
Fortunately, you can have the IT group install any number of different browsers without impacting IE in any way. Here are some links: