In reality, lots of reasons. While there are a lot of applications that don't work well with IE, there are a bunch of others that don't work well with anything BUT IE. A lot of SharePoint-related applications being the biggest example but there are others (a lot of features of our internal Portal site will only work in IE). IE also offers a cleaner ability to do SSO passthrough credentials (you can do it in Chrome/Firefox, but not without GPOs and in some cases 3rd party plugins) Also, you have complete control over patching IE where as Chrome updates when Google chooses to update it. The lack of control can be worrisome, especially if, for example, Chrome has an update that breaks a critical web app and you didn't have the ability to have QA test it because it updated automatically before you even knew it was being installed. Chrome also can be very network unfriendly (it's so fast because it keeps a lot of connections open that IE would traditionally have killed). And while you can control it (while introducing other limitations), the ability to have 3rd party plugins for Chrome and Firefox can create more issues as well (as some of these are malware/spyware).
Not trying to be an IE evangelist, just saying from a pure IT perspective, it isn't cut and dry. Some newer, less-legacy oriented companies can completely eliminate IE and good for them. The flip side is I have been at companies that to this day have apps that can't run in IE9 or higher (thank goodness IE11 introduced the Run In mode for these cases). As an avid Chromebook user, would love to be in a Chrome only world, just a lot of places aren't there yet and it is easier to make IE "corporate policy" so that you know what the experience will be.
