Question

why have the western slopes of Western ghats covered with thick forest and not the eastern slopes​

Answer 1

It is because the western slope gets much heavier rainfall than the eastern slopes. Moisture laden air moves eastwards across, the Western G hats during the monsoon season.

Answer 2

The western slopes of the western ghats happen to be the windward side of the western ghats where they receive heavy rain due to the southwest monsoon or the summer monsoon... thus on the western slopes there happens to be forest that require lots of rainfaall while the eastern slopes being in the rain shadow area receive very less rainfall thus contain no thick forests..