iCloud hosts the calendars on its server and your various computers read them from there. The protocol for doing this is called CalDAV.
Outlook for Mac has not been equipped with the programming for CalDAV and therefore cannot access iCloud calendars. Outlook for Windows evidently has and can.
This is something only Microsoft can rectify and you could send them feedback suggesting they do this with all convenient speed.
Outlook on a Mac can only access calendars living under 'On My Mac' and these cannot sync with iCloud. iCal can sync with iCloud provided you have 1-.7.2 or higher.