Why are we unable to add Requisition Tokens to IRC_Candidate_Merge_Status notification?

We added the below tokens to the IRC_Candidate_Merge_Status notification in the Alerts Composer. Why can't these tokens be added to this notification but the tokens can be added to every other IRC notification in the Alerts Composer?

${RequisitionNumber} - ${RequisitionTitle}

It results in 'null - null' in the Subject and Message Text of the notification.