It could be packaging thing. A thinner cable can be bent more tightly and takes a lot less room in the box. Though I figure you could wrap the cable around the phone or something like that if it was thicker.
Or, like you said, it could be a cost saving thing.
Certainly just cost savings, just like removing the charger was (reducing waste is nice, but it's just a pretty excuse, apple chargers were uselessly weak anyway - they included 5w charger with iphones, ridiculous - but the others generally weren't).
They also could take a lesson from Ikea on how to properly pack stuff, one of the points in the excuse was that it made packages flatter but they could do that by moving the charger to the bottom of the phone instead of bellow. They could do this and more with the cables, circle it around the phone could make for much smaller boxes