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.