Why are you not allowed to overtake here?

Your answer is fully correct.

You are not allowed to overtake here for two main reasons under German traffic rules:

1. The side clearance to the cyclists is too small. Since there are several cyclists riding close to the edge of the lane, trying to overtake in this narrow space would risk hitting them or forcing them off the road. Ensuring enough safe distance to vulnerable road users like cyclists is mandatory.

2. You may endanger oncoming traffic. Although you might see the road ahead, it is curving and there could be oncoming vehicles hidden from view or just around the bend. Attempting to overtake in such situations is risky and prohibited.

The third option about not crossing the center line is not the primary reason here because the road markings allow passing on your side, but safety concerns about the cyclists and oncoming traffic take precedence.

In summary, when overtaking, always ensure enough safe side clearance to cyclists and do not overtake when oncoming traffic poses a danger, especially on curves or narrow roads. This helps protect all road users and prevent accidents.