The reason is that people don't follow directions and that controller doesn't have the ability to sense when the pump is disconnected. It requires more logic. They experimented with the removable connection early on when the RD3 80's were released in Europe. People were unplugging the pumps from the controller when the power was connected and or plugging the pump into the controller when power was connected and there were issues with the controllers failing as a result. Because of that, Germany made the decision to err on the side of caution and not include the quick connect on that controller/pump combo. The RD3 150's and RD3 230's use a more advanced controller that added some safety features to help mitigate accidental damage from hot plugging the pumps. This is why we see that feature in the bigger pumps but not presently in the smaller ones. It's a more advanced and more expensive controller. Despite the connection on the 150 and 230, it's obviously stressed that you don't hot plug the pump while power is connected to the controller.
I have an RD3 80 on one of my tanks and the RD3 50 skimmer pump on my main display. I just bring a container with some vinegar in it and another container with water. I break the pump down right beside the tank and do a quick vinegar soak followed by a scrubbing and rinse. The whole process takes me less that 15 minutes including the 10 minute vinegar soak. Sometimes I pull the skimmer controller off the cabinet door but not often these days. It's just too easy to do it right by the tank and not have to undo the long pump cable and then retie it back out of the way when I'm done. Even my 230's with the removable connection get cleaned right by the tank for that reason. It's just easier to clean them there than to take my nice neat wire bundles off the wall and carry the pump away when I can clean them within feet of their location.