I suspect a bunch of technical debt - the core of the application dates to 2000 or before (there are menu's changed for a very very long time).
It was supposed to be Outlook 95 and ship with Office 95, but the app was late.
I know at least one CEO who will absolutely explode if they fuck up his Outlook. He probably have 20 years of business correspondences in there. Is that a good idea? Probably not, but he does. If Microsoft mess up Outlook, then there's very little reason for his company to not look at Google and Gmail, and he probably will just because he'll feel like Microsoft screwed him over.