Okay, a 1 ton battery is flying 5" over the hwy at 80 MPH. It's protected by 6mm of aluminum, but here comes a 6" trailer hitch. Bang. scrape. Fire.
Now what happens when you replace the aluminum with steel armor? You get a bigger bang before the first starts. That's pretty much all. There just isn't room under that car for the hitch to clear.
You'd be better off trying to put some kind of cow-catcher type of grate on the front of the car, to deflect debris away from the battery, but you are still talking about a heck of an impact. (and a hitch that is launched sideways at ~140 MPH by the impact!)
Model S is already a pretty heavy car. Adding more weight means increased cost, weight, less maneuverability and decreased range. Do they even have room at the bottom of the car to do this? Wouldn't that mean a higher center of gravity too?
Replacing with steel would be about 500 pounds more weight. As you point out performance and milage would be effected. They made a strategic decision to go with the Battery pack under the car and now they are paying for it.