Why does the rainfall decrease from east to the west in Northern India?

The rainfall decreases from east to the west in Northern India because monsoon rises from both the Arabian Sea and the Bay of Bengal, which causes heavy rainfall in the Brahmaputra valley and the Assam hills. As soon as they move toward Himalayas they take western turn and move up the Ganga valley, but due its westward approach it become drier and drier resulting in lesser rainfall North West India. The shrinking moisture levels results in decrease in rainfall from east to the west.