It is not that it is close to India, hence it is visible. There is actually a maritime law behind showing Sri Lanka in the Indian map.

If Sri Lanka is not shown on the Indian map then it well be considered a legal offence.

This is the reason why Sri Lanka is definitely shown in every Indian map. This law was made by the United Nations. 

 According to law, if a country's boarder is boardered by sea. So the maritime area up to about 370 kilometers will be considered the maritime territory of that country. 

Sri Lanka also comes within this maritime border of India.