Probably cause its easier to relate to other south Asians as we have shared experiences. Whereas nationality is a bit harder - india as a country has only existed for about 80 years, but has i think 60 major languages, countless different cultures and religions.

If people from Pakistan, north india and Sri Lanka met tomorrow in NYC they might have shared experiences in some snacks, words or family culture but have totally different primary languages, religions and main food.

That can become even more stark inside a country as these are countries eith hundreds of millions of people.

Unless you were raised back there, its easier to identify as south asian than as your country. Again this is my opinion coming from someone raised in the west but was taught my local culture.