Why are Email Merge fields not populating on a Salesforce Email Template?

Salesforce Email Merge fields will only populate if there is a Contact Specified.  Salesforce Email
If unable to specify a Contact, please review if a Drawloop Email Template will meet the use-case needs: Email Template

How to Specify a Contact?

Example: &contactId={!Object.Contact.Id}

Specifying a Contact in the Button
 

 

Option 1: 
Change the Email Template from a Salesforce Template to the Drawloop Email Body Template.
Drawloop Email Body Documentation
** Note: With the combination (Drawloop Email Template + Internal Email + Autosend) No Activity History will be created: https://community.nintex.com/t5/How-To-s/Drawloop-Send-Emails-without-Creating-Activity-History/ta-p...

Option 2:
(If a Salesforce Email Template is desired, Salesforce requires a Contact Id when auto sending. If Internal email is checked, it removes the Contact and uses the running User as the Id)
Uncheck the Internal Email checkbox and add a Contact Id to the Configuration.
1. Contact Selection on the Run Page.

2. Button Parameter &Contactid= to the Button. Documentation: Button Reference
3. For Drawloop_Next fields, add SFContact: with the appropriate Id or Id field

 

Salesforce Documentation: https://help.salesforce.com/articleView?id=merge_fields_email_templates.htm&type=5

Specifying a Contact in the Button: https://help.nintex.com/en-US/docgen/docservices/docgen-sfdc/Services/setup/ButtonWizardReference.htm