Because unlike Firefox or Chrome, Microsoft has thousands of enterprise customers who use IE to host intranet applications that depend on specific behaviors of

Because unlike Firefox or Chrome, Microsoft has thousands of enterprise customers who use IE to host intranet applications that depend on specific behaviors of the browser. Thus, Microsoft is essentially required to freeze development on a browser prior to its release to ensure stability of behavior (not the same thing as debugging) and to take steps to ensure that the new version can properly emulate the behavior of older versions (i.e., incompatibility mode and the previous browser modes). Firefox and Chrome don't even try to cater to enterprise users, so they can focus on releasing new versions every few weeks even if those new versions break websites or addons (which happens quite frequently with both Firefox and Chrome).