Why are you not permitted to overtake in this situation?

You answered correctly. In this scenario, overtaking is not allowed because there are two important risks: First, the presence of oncoming vehicles, including a motorcycle, means you do not have a clear and safe stretch of road to pass without endangering yourself or others. Second, if your vehicle cannot gain enough speed quickly and safely to overtake the slow-moving tractor, you risk remaining in the oncoming lane for too long, which can cause an accident. German traffic rules require that overtaking is only permitted when you have a clear view ahead, enough space, and the ability to overtake quickly without hindering or endangering other road users. Always wait until the oncoming lane is completely free and you are certain you can pass safely within the available distance.