It's Microsoft not being compatible with everybody else. Microsoft had the chance to jump on open standards during many upgrades of Exchange, even for mail. That's what Apple did.
If you are on iCal and Apple screws you over, migration to any other package using the same standards is easy. Not so for Exchange, you're locked-in, still paying dearly for those CALs.
It’s easy to jump on a new standard when you don’t have any exiting standard it might conflict with. “Tearing it all out and starting over” doesn’t cost you anything when you never had anything to tear out in the first place. What kind of enterprise email/calendaring solution did Apple have to convert when they signed up for the new “standard”?