Because candidates are not yet confirmed as attending the school, they are not assigned a Grad Year as they would have no reason to have that Alumni Affiliation if they don't attend the school.

Once a candidate has been enrolled through the Enroll task in Enrollment Management > Admissions > Admissions Management > Enroll/Inactivate they are automatically assigned a Grad Year based on the difference between their Entering Grade and the highest available Grade Level at the school.
Note: Grad Years can also be added manually to a candidate through Core > People Finder > Access tab on User's Profile > Alumni Affiliation in the Personas block.  If a candidate has a Grad Year when they are enrolled, the automatically assigned Grad Year will overwrite the existing one in the candidate's profile.