As others mentioned one part is the cost of living. Some places are cheaper to live in than others. Thus, even in India you might get more salary in a bigger city than in a smaller city.
But, bigger issue is the one of economic barriers.
In an ideal world, labour and products would move around easily bringing out constant wage everywhere for the same labour.
In fact, in Europe it is somewhat true. Wages across UK, France, Germany and other western European countries are very close as there are no economic barriers. Even when the Liras, Marks, Pounds and Francs had widely different exchange rates.
However, between the developing world and the developed world there are huge barriers. These are through import duties and visas.
Balaji Viswanathan, CEO of Invento Robotics