My guess, and this is just a guess, mind you, is that the the long roof attached to the D-pillar, which itself is attached to the unibody, adds structural rigidity to the car and allows it to tow more and to support more tongue weight without deforming the body of the vehicle. A 300, Charger, or Challenger would lack the high structural support. And while the modern Challenger is arguably a two-door-post sedan rather than a coupe (based on the roofline), but its shorter wheelbase could also have an effect on the ability to handle a trailer of any significant size.