Most of the ones like that vent the heat primarily through an area that can be effectively cleaned and has significantly better airflow. So the back radiator is larger but it's effectively never cleaned and the air exchange volume is the gap between the back of the fridge and the wall.
Venting out the bottom allows exchange with the rest of the room, but does require more work to ensure the air actually flows. Additionally, you can clean the vent with a vacuum, and it's a lot more likely that will actually happen than someone pulls the fridge forward and wipes the coils with a damp but not wet cloth.

I would imagine not having to clean the radiators.

Those things are a bitch to clean, and they get disgusting quick