A Subcontractor is a special case of a Vendor. 

There is a field on the Subcontractor card called Vendor Type that can be "Vendor Only", "Vendor and Subcontractor" or "Subcontractor Only".

If you create new subcontractor and set the type as "Vendor" it will appear on the standard vendor list only. 

To remedy this open the Vendor card and change the Type to "Subcontractor Only" or "Vendor and Subcontractor".