
This style of dog cage is better suited to cars with angled tailgates

As shown in this Mercedes, the dog cage is contoured to the angle of the tailgate giving extra space inside the cage, that would otherwise be lost.
Even in cars where the angle is not required the drop down flaps are a custom option as they can protect the bumpers and also provide a "step-up" for dogs into the cages - as this Mondeo shows.
The same reasoning can also be applied to tailgates, as this tailgate in black powder coating for a BMW 3 Series touring shows.