Why are you not allowed to overtake now?

You answered correctly. In this situation, overtaking is not allowed mainly due to the presence of the motorbike (which represents a slow-moving vehicle ahead) and the layout of the road (such as curves or limited visibility that make overtaking risky). According to German traffic rules (§5 StVO), overtaking is only permitted when it can be done safely and without endangering others. If the road layout, like a curve, limits your view or if another vehicle (in this case, the motorbike) is creating a potential hazard, overtaking would be unsafe and therefore not allowed. It's important to assess both the road conditions and the traffic ahead every time before deciding to overtake.