Why can't runway 18 at Frankfurt Airport accept takeoffs or landings in the opposite direction?
Let us consider the operations other than takeoff on runway 18.


Takeoff Runway 36

There are buildings not far off the departure end and directly after the end of the runway is a movement area and ramp.  To safely takeoff they would have to clear that area and probably replace all the concrete with crushable concrete (more on this later).  The buildings probably post obstacle clearance issues for engine out climb performance.  The east-west runway north of 36 would have conflicting traffic and would have to be carefully sequenced.  Lastly, that purple area to the north of the runway is probably a noise sensitive area you aren't allowed to fly over at low altitude.
Landing Runway 18

You approach over a noise sensitive area, cross over operations on the northern east-west runway, overfly a building at extremely low levels (wheels on the roof) and have to make sure there are not airplanes on the ramp between the building and runway.  Landing short would be nothing short of disastrous and every landing would be a risky operation with the close proximity of airplanes and buildings.  Glideslope services would probably not be available due to ground obstacles.
Landing Runway 36

This is the least problematic approach.  The problem is in missed approaches and long landings.   There is no safety area at the end of 36 and a runway excursion past the end of the runway would be catastrophic.  To mitigate this they could re-pour the ramp concrete with crushable safety concrete, but that would probably make using ramp and building unsuitable.  If traffic goes around they conflict with aircraft on the northern east-west runway and have a noise sensitive area to deal with.


In comparison to the above, taking off to the south only requires a blast fence just north of the runway to protect the ramp and building from jet blast.  There are no logistic issues in aircraft movement on the ramp and the building is not in danger of being hit.