Question

why most of the deserts are located on the western margins

Answer 1

because the prevailing winds in the tropics are tropical easterly winds. The tropical easterly winds become dry by the time they reach the western margins of the continents and so they bring no rainfall. Thus, the region becomes devoid of moisture which causes dry conditions leading to formation of deserts.    

Answer 2

Most of the worlds deserts are located in the Western margins of continents in the subtropics because the prevailing winds in the tropics are tropical easterly winds . The tropical easterly winds become dry by the time they reach the Western margins of the continents and so they bring rainfall