This simple video says that there were 170 million people in the world at the beginning of the current era. Each dot represents a million people -there are about 170 dots on the map- and over 120 of them are in Northern China and India. The video portrays about 70% of the world's population residing in these two regions. It can be assumed that the majority of the people resided in these regions. There was nowhere else like these until Europe around 1500 A.D. When did Northeast India and NE China begin to dominate in global demographics?
Nothern India and Northern China are both very fertile places that would support a large population of farmers or hunters. This was probably a good starting point, but population density triggers societal advancements that increase the population more. I think that it would be not simply fertility, but this "snowball" effect that created the concentration of population portrayed in the video by 1 A.D.
Discussions on global population generally start at this point because earlier data is not available. The consolidation of China and Rome created useable census records. Earlier statistics just aren't there, making the discussion theoretical, and estimates varying often by 200%. There are some older census records in China, such as from the Zhou dynasty (c. 900 B.C.), that can't be reliably used.
Pieter Geerkens had a good answer. He said that rice has a caloric advantage over other types of grains, and that the the Ganges river system is the one of the largest, most fertile places. The user Rahul had a good point; that the development of river systems facilitated social development. Still these are all partial answers, and I'd like to bring up some other points.
The date that Pieter Geerkens gives for the adoption of rice cultivation in India is about 2000 B.C. There are issues with considering this as a starting point. This period of history in North India is called the Late Harrapan phase; it is characterized by the collapse of the Indus Valley Civilization, and a shift to more rural society. Regardless of the character of society, though, both the IVC and the Late Harrapan cultures were located in the Indus River system, not the Ganges river system, which was the region of great density shown in the video. The Indus Valley is shown with a more typical density like that of the Mediterranean. Being that it was the region of the previous Indus Valley Civilization, it seems like it would be the site of the greatest population in North India at the time. This picture of Late Harrapan archaeological sites shows the regions associated with it, as well as other archaeological sites in North India in non-bold text.
Due to this black hole of easily available information, I'm inclined to think that the Indo-Aryans were the ones who developed the Ganges River system. Vedic culture was initially focused in Northwest India, and moved into the Western Ganges river system around 1200-1000 (Painted Greyware). Some time after 800, the development of the iron plow revolutionized agriculture in North India. Moving later to 600 B.C., you have the development of long distance river trade; Adam Smith's criteria for civilization, and the hegemony of Maghada. Maghada was centrally located in Northern India, on a fertile plain where two major rivers almost converge.
I'm inclined to think that what was the classical era in the west, was a time of great growth in the civilizations of the east. The iron plow revolutionized agriculture by allowing for deeper cultivation. This fast maturity could be shown by the rapid creation of the Mauryan Empire in 344 B.C.
I know nothing about Ancient China. What is called the Warring States (475-225) period did produce a lot of cultural works, so it may be mischaracterization of the period.
Conversely, it could be argued that it was simply an environmental phenomenon, pushing the answer far back into prehistory Such a viewpoint would have to ignore the fact that these two places happened to be the two centers of eastern civilizations, and fail to explain the lack of similar examples elsewhere.