Event merge fields can only be pulled into an email if, at the time of creation, the event is selected to be associated with the corresponding event.

How to create an email campaign linked to an event: 
1. Navigate to Communities > Email
2. Click New
3. Under Campaign, select your campaign
4. Under Associated Initiative, select the initiative type
Note: after doing this, you may have to reselect your campaign in the Campaign section
5. Under Initiative, select your correlating initiative
6. Complete the remaining steps in creating an email campaign