A combination of the right humidity level and temperatures. They thrive on the chalky soil here.
All pictures taken in Saint Julien de Lampon, on the border of Dordogne and Lot. We used to live here for nearly 10 years, and I never saw even one orchid.
From top to bottem:
Orchis Mascula
Orchis Purpurea
Orchis Ustulata
Orchis Araignée