Question

why do the cyclones largely confined to south China Sea. Bay of bengal

Answer 1

This could be an answer 

Tropical Cyclones are low pressure point and  winds from surrounding high pressure area moves and rotates around the low pressure point. The Tropical cyclones originate in Tropical region (5 to 30) in both hemisphere. Origin of tropical cyclones need warm water bodies where temperature is more than 26 degree Celsius. Continuous supply  of warm  moisture is needed for the condensation in upper atmosphere which releases more latent heat of condensation, this process will result into vertical column of low pressure. For the the formation of this vertical column, calm wind condition is needed. The above conditions get fulfilled in South China Sea , Bay of Bengal, Gulf of Mexico and other tropical oceanic regions. 
Diameter of tropical cyclones varies from 5 to 300 km and it moves from east to west. It may not reach coast line and gets dissipated in ocean itself, if it has to move long distance from point of origin to coast line. As the east coast of continents are near to the mentioned regions so cyclones could reach there. Thus these east coastal regions has more cyclones than other tropical regions.