Well, that mainly has two reasons:
1. IE versions have major differences
While other browsers may have no (obvious) difference between versions, Internet Explorer, being the only browser pre-installed (and basically hard-coded) in Windows, has huge differences from version 6 to version 10. Version 10 is almost as good a browser as Chrome or Firefox, while version 6 is an unreliable, slow, good-for-nothing, over-customized browser still used by some non tech-savvy, and it is incompatible with thousands of features introduced after it was created (that was over a decade ago).
You can see some compatibility examples here.
2. Being pre-installed has an impact on the market
Since IE comes with Windows, and while other OS are gaining up publicity, Windows has been the default for thousands (if not millions) of people, for a long time. Since these people hire programmers to do stuff, like make their websites, programmers are forced to make it look good on the client's screen, even if that doesn't always target the largest audience.
Of course, most of us are trying to have a good result on both the client's screen and their clients' screens, but that isn't always easy, if our client has IE 6. (And believe me: some of them will think that you are not a good developer if you ask them to change their browser)
So, in conclusion, we tend to always refer to IE with its version, because it does mean something different for development.
P.S.: Here is a great blog article about the history of IE and why geeks hate it which does a great presentation on a once good browser.