In Alberta, we do have farms in all geographical locations but you are correct that most of the dairies are located on the prairies.
The main reason why dairy farms are located on flat ground is related to farming practices. Growing crops for grain, silage and hay for the dairy operation is important to the dairy farmer and growing crops efficiently is best on flatter ground.
Also, in Alberta the soil types best suited for crop production are located on the prairies. In southern Alberta where the terrain is the flattest, farmers also have the benefit of using irrigation. Irrigation is best suited to a flat terrain. The rougher terrains are better suited for beef cows that graze all the time whereas dairy cows are usually housed in barns. Learn more about dairy barns here.