Lack of integrated email/contacts/calendar?
I can't believe that's really the reason. I can understand that migrating FROM Outlook might be a pain for a lot of companies, so they suck it up and stick with it. But if you're not tied to MS ecosystem already, why would you willingly adopt it?
There are a ton of solutions, both Open Source and proprietary. Gmail, Zimbra, and Evolution, just off the top of my head.