Why do you have to slow down here?

You answered correctly. The reason you must slow down in this situation is the presence of the motorbike (in this question, motorbike refers to a vulnerable or slow moving vehicle, but the provided correct answer is the one for "motorbike"). In real-life traffic situations, and especially in German road rules, you should always slow down and be ready to react when encountering vulnerable road users, such as motorbikes, bicycles, tractors, or pedestrians, especially when passing them or sharing a narrow road. Slowing down allows you to react safely if, for example, the motorbike swerves or stops suddenly. This rule helps prevent accidents and ensures the safety of all road users. In similar questions, always consider whether a vulnerable or slower vehicle is present and if the road situation might require you to reduce speed for safety reasons.