South Asia is a really difficult place to try and centralize the countries. A big problem are the large populations in the countries, with the likes of India, Pakistan, and Bangladesh all being in one of the most populated countries in the world, so centralizing them would mean that hundreds of millions of people will either live on the verge of existence (even worse than it is now), or will be moving towards the city where the centralization will be taking place, and there's no such place that can function properly if that amount of people flood in it. Another problem is that there's multiple cities that are very big, with millions of people living in them, and if centralization takes place, they will start to decline in every aspect, and that will cause huge problems of all sorts on national level.