Hi OezAli,
Outlook.com and Office 365 are different services. According to the information in the link below, for Outlook.com account, with the IMAP connection calendars can’t be synced to the Outlook client, as David said above: https://support.office.com/en-nz/article/Add-Outlook-com-to-Outlook-for-Mac-2011-fde8325c-963f-41db-bd19-804dd05c11cc .
Since we have limited resources about Outlook.com account here, I recommend you contact outlook.com support to see if they could offer any workaround to get the calendar synced to Outlook 2011 for Mac. Thanks for your understanding.
Best Regards,
Vivian Wang