Why are you not allowed to overtake here now?

You are not allowed to overtake because of the motorbike coming from the opposite direction. According to German traffic rules, you must never start overtaking if there is oncoming traffic close enough that it would be unsafe to pass. Even if the road looks clear ahead, the presence of an oncoming vehicle requires you to wait until it has passed before you can safely overtake. This rule helps prevent head-on collisions and ensures the safety of all road users. So, when you see an oncoming vehicle like a motorbike, you must stay behind the slower vehicle until the road is clear.