Why may you not exit now?

You answered correctly because the presence of the cyclist approaching quickly from behind is the main reason you should not exit the current lane or position. In German traffic rules, you must ensure that it is safe before changing your position on the road—this includes checking for faster vehicles like cyclists who may be overtaking you. A cyclist can be in your blind spot or close behind, so exiting or moving without checking mirrors and waiting could cause a dangerous situation.

The motorbike coming from the opposite direction does not restrict your exit since it is on the other side of the road and not affecting your lane or space. The pedestrian on the sidewalk also does not prevent you from exiting because they are safely off the road and do not interfere with your lane.

In summary, the key rule is to always be aware of vehicles behind you, especially bicycles, before changing your position or exiting your lane to avoid collisions. That is why the cyclist is the decisive factor here.