History lesson...
Here's a little history lesson; a long time ago, on a PC far, far away (ok roughly around the turn of the millennium on PCs everywhere), two companies, one known as Microsoft and another Netscape were building web browsers (so were Opera, but they were charging for it so nobody used it, amongst others).
MS owned the OS space with Windows, munged IE into Windows and killed off the competition, giving themselves a 95% market share in the browser space - irrespective that this was an illegal, anti-competitive practice, that's what happened.
Once they'd got the browser market wrapped up, Microsoft were effectively the defacto owners of the standards - so many, many monkeys in web-code land coded to the MS "standard" - and MS saw that it was good and decided to stop there; IE6 would be the LAST web browser ever, the HTML/CSS/JS standard was set in stone forever.
Now those many, many web monkeys made many, many websites and intranet applications all in-line with the MS standard... then from the ashes of Netscape arose Firefox. Firefox (and virtually every other browser) supported the W3C standard NOT the MS "standard" so as Firefox gained traction, by default, the percentage of people browsing the web using the W3C standard increased - the glaring errors in IE6 (such as the box model) became more apparent in poorly written websites that adhered to the MS way of doing things.
Until today - where the IE6 legacy plagues the business space - old, shonky intranet applications that are no longer supported or developed but still widely used sit on the network, applications that will only work in Internet Explorer (as they were coded for "the last ever web browser", IE6) - so the IT department HAS to mandate the use of IE6 for those applications - just because some lazy arsed web monkeys couldn't be bothered to code to something resembling the (admittedly somewhat irrelevant at the time) W3C standards and implement kludges for IE6.
If they'd done it right in the first place - we'd not have this issue now. It has ALWAYS been possible to code cross-browser/cross-platform, although with a lot of parallel JavaScript code... and had they done that the applications would work in browsers today, even the (much improved) latest version of Internet Explorer, IE9. I know, I've got stuff running where the UI code hasn't been touched in best part of a decade - though I may update it to make use of some of the CSS3 features at some point.