The use of the symbol came after Constatine and it appears after cruxifiction pretty well ended. It was an expression of the Roman Pagan State in particular and it targeted the non elites and slaves in particular. All good reasons to not promote the image. After all that common state execution was the simple noose, or a sword cut or even a knife. Also the advent of Christianity made creating a painful death less acceptable. Today we actually try to make it as painless as possible. Again this is a result of our now fully globalized christian legacy. Who would have thought.