I'm not sure about this but a lot of software might have been written specifically for IE6 and IE6 only, which might not translate well to Chrome/Firefox, and could cost businesses a lot of money, more money than they deem its worth. There could be ActiveX controls and such which can't be ported to Chrome/Firefox without money.
This is based on my experience at UPS which used a single machine using IE6 and some form of web app to control start up and shut down procedures, or something to that effect. It had no Internet connection, only LAN I believe.
I did ask myself this question too and it does seem peculiar. After some thought, my only guess is the above!