Why are you not allowed to overtake now?

Your answer is correct because overtaking is prohibited in this situation due to the presence of a motorbike (or in general, a vulnerable road user like a cyclist or slow vehicle) and the layout of the road. According to German traffic rules (§5 StVO), you must not overtake if doing so would put other road users at risk or if the road's design makes overtaking unsafe.

Specifically, the road layout—such as curves, narrow lanes, or limited visibility—can prevent you from maintaining the legally required safe lateral distance (at least 1.5 meters when overtaking cyclists) and clear sight of oncoming traffic. In this case, trying to overtake the slow vehicle would endanger the motorbike or cyclist present and possibly cause a collision.

Therefore, you must wait until the road ahead is clear, wide enough, and it is safe to overtake without endangering other users. This ensures the safety of all road users and compliance with German traffic regulations.