Why may you not exit now?

You answered correctly: you may not exit now because of the cyclist. In German traffic law, when you plan to exit a parking spot or move into traffic, you must always check for other road users approaching from behind, especially faster and less visible ones like cyclists. If a cyclist is coming up from behind, you must wait because exiting could put them in danger or force them to brake suddenly. The presence of a motorbike coming towards you in the opposite lane or a pedestrian walking on the sidewalk does not prevent you from exiting, as they are not in direct conflict with your intended path. Always check mirrors and blind spots carefully for cyclists before pulling out.