Answer: Because of the weather.
Step-by-step explanation:
In the North of Mexico, the prevailing ecological conditions are not favorable for the realization of agricultural activities, the quality of the soils and the characteristics of the climate considerably limit the natural availability of suitable land. Mexico is crossed in its middle part by the Tropic of Cancer that marks climatic differences between north and south. The high mountain ranges that develop parallel to the littorals of the Pacific Ocean and Gulf of Mexico work as meteorological screens that prevent the free passage of humid winds from the sea. Because of that in northwestern Mexico (Baja California, the coast of Sonora and Sinaloa) dominate climates according to the climatic classification of W. Köppen type BW and BS, arid and semiarid climates.