The rainfall decreases from west to east in northern India because the temperate cyclones which develop in the Mediterranean Sea move along with the Westerlies and bring rain to Northwestern parts of India. So when we move towards eastern part, the rainfall gradually decreases.
Monsoon wind is divided into two branches which brings moister and cause rain-fall across the Indian sub-continent. First south-west branch exhausted its most of the moisture(coming from Arabian Sea) raining over western ghat. When its one branch reaches to western part of north India, it is left with little moisture. 
Second branch of monsoon from south-east brings moister from the bay of Bangal and its one branch causes rain in north-east and other moves towards west. The westward flowing monsoon wind causes rain in northern plan. As it moves towards north-west it gets deficient of the moisture, so rainfall decreases from east to west in north India.