Island kingdoms /countries are very proud of themselves.Though they are not far from mainland they think the borders of their countries are natural and very clear. Japan, England,NZ are some examples.
If unique religion, language and culture added to it then they are permanently a separate country. SL has a combined mythology,history and mutual cooperation with India. But it's Island geography(It has about 20 ancient and modern names including Ceylon,Suvarna dvipa,Sinhala,Ilam), far away from all other nations except India, Sinhalese language, Buddhist religion and culture(Men and women are equal in status from politics to dresses since thousand years!) make it feel and be like a separate country.
And the shrewd British saw it to that SL remain a free country by granting it freedom in 1948 where as India got it in 1947.
You must note that never did once people of SL declare of joining India, though Nehru had thought once.
Still the geographical map of India is not complete without SL and BD. It is because they come in between.
Emblem of Sri Lanka.
Photo internet.