India is often called a subcontinent because it is a distinct landmass, not just a country.
A subcontinent is a part of a continent that is politically and geographically separate from the rest of the continent.
While it has many features of a continent, it is not as big as one, so is not considered a continent.India is now joined with the continent of Asia but is a distinct area separated by the natural frontiers like the Himalaya’s in the North and the Deccan Peninsula in the South giving it a unique identity on the globe.
Originally it was a separate geologic plate but has collided and merged with Asia due to continental drift.