I would presume to protect their faces from flash burns, a clawing suspects' fingernails, or other hazards. That's just a guess. I don't know their policies and procedures.
Why don't the officers making traffic stops wear them?
Perhaps there is no reason to.
All that said, if it wasn't for the policies and laws that allow and promote this activity, I dont think we would have officers engaging in it. So my real problem is with our government that believes it has a right to subject its citizens to such treatment on a mere suspicion of (minor)wrongdoing.