Another big reason is Active Directory. It's very surprising to me that there isn't a full-featured FLOSS alternative to AD, because what it does is pretty simple, but it works pretty much out of the box, and it scales massively.
Two years ago I would have said another big reason was Outlook, because no other desktop calendaring app comes close (even though it sucks hairy balls as an email client), but nowadays web-based solutions, particularly Google Calendar, are as good if not better.
Two years ago I would have said another big reason was Outlook, because no other desktop calendaring app comes close (even though it sucks hairy balls as an email client), but nowadays web-based solutions, particularly Google Calendar, are as good if not better.