[rsrguy]

A snow covered hill without measures doesn't provide a consistent surface to jump from for all competitors.
Every time a ski runs across snow, the surface is melted. In cold conditions the water freezes back into the snow, with temp fluctuations during an event there's too much variation in the track to provide a consistent surface...this is why the alpine event courses are all injected with water. Ice is a more consistent surface than snow.
The jumps are even more subject to consistency issues because everyone runs in the same track. On an alpine course the line can vary a bit, but even then they always rut out in the same places.