It doesn't work because the side face of the feature is planar to the side face of the box.
Syncronous can't appear determine the boundary where the feature and box join.
It is the same if you try to create a circular pattern of a similar feature around a cylinder - if the end of the feature is planar to the end of the cylinder it doesn't work. One of the limitations of sync.
Just to clarify - I am using ST7, so things may have improved in later versions.
EDIT: Sorry, the cylinder example I gave now appears to work - didn't used to though
I'd recommend using the Selection Manager to recognize a Rib/Boss from the face and then mirror it as below: