Supporting IE carries massive baggage as you have to pin yourself to Microsoft's erratic, tactic based release cycles and limiting yourself with future features. Chrome and Firefox now how have rolling releases and are closely tied with the developer community, so new features and rendering uniformity is what you gain.
There's also the ethos you buy into by supporting IE: you're supporting a closed source, proprietary internet.
By your reasoning, nobody would write an iPhone app either, or anything targeted for a platform other than GNU/Linux.
Microsoft has played many sneaky pete tricks in the browser wars (such as supporting a CSS standard that couldn't be correctly implemented in Netscape's codebase) but you can't blame their release schedule entirely on that -- Windows users tend to be a conservative lot who are terrified that Microsoft will push something that will break their cheap-ass intranet that's held together with string and ear wax.
Due to...everybody say it with me...lack of standards support.
Mozilla has had similar issues with intranets (though not to as large an extent, due to their smaller install base), and you can't exactly blame _that_ on lack of standards support. What you _can_ blame it on is things like people writing intranet apps that depend on the user flipping some hidden pref in the browser to relax a security policy and then breaking when that pref is later removed, say.
If an institution sticks doggedly to one version of one browser, then it stands to reason any intranet software is going to be optimised for nothing other than that.
Because they employ more evangelists? The IE team has spent a tremendous amount of resources informing developers about IE 9 and 10. Many of us just don't bother paying attention to them.
> There's also the ethos you buy into by supporting IE: you're supporting a closed source, proprietary internet.
The irony here is that you're supporting a closed Internet by ignoring browsers. What a wonderful luxury that is.