Why can you not turn off the road here?

You correctly identified the reason why you cannot turn off the road here. In German traffic rules, you must not pull off the road or turn into a driveway or side street where your maneuver would endanger or obstruct other road users. The presence of the red car means that turning off now could cause an accident or force the car to brake suddenly. Therefore, your driving priority is to wait for a safe gap before turning. Understanding that you must consider all surrounding vehicles, especially those close behind or approaching, helps ensure safe and smooth traffic flow.