Very recently, ESA released a new video showing the proposed Ariane 6 rocket being assembled. This mostly takes place in a large 'moveable gantry' building where the payload is placed on top of the rocket and boosters are added. This moveable building also has different platforms that come in from the sides and "hug" the rocket, presumably for technicians to access it. When the rocket is ready to launch, the whole building moves away leaving the rocket on the launch pad and ready to go.
How does ESA (or Arianegroup) justify the costs of a vertical assembly system? I can't imagine that constructing a huge moving building is cheap and the Soyuz and Falcon (including heavy) are both assembled horizontally and of similar size. Is there some deal-breaking requirement for vertical assembly? Or, am I wrong in assuming that vertical assembly is more expensive?