Question

Why are the western slopes of western ghats covered with thick forests and not the eastern slopes....?

Answer 1

because the western ghats get heavy rainfall while the eastern part is in the rainshadow area so it does not recieve rainfall

Answer 2

It's because the western slopes get much heavier rainfall than the eastern slopes.  The westward facing slopes receive much more rain than the eastward facing slopes and the Western Ghats are considerably wetter than the dry Deccan to the east.