Why ganga plains are more populated
Answers
I will just make it simple.
Indo Gangetic plain gained its importance since Tertiary period.
1) Indo Gangetic plains provided the basic human needs (Food, water, clothing and Shelter) without any additional efforts. Most of the early civilizations like Harrapan/Indus valley civilization started due to this reason.
2) Indo-Gangetic plain gained important because fertile alluvial plain supports Agriculture Activities.
3) Availability of Fresh water because major rivers like Ganga, Yamuna & Brahmaputra flow in this region.
4)Major battles took place in this region because Invaders like Aryans and Mughals who came through Khyber pass found this place suitable for their settlement.
5) During Tertiary period, Gangetic plains were adjacent to Himalays(Rocky Terrain) and Deccan plateau (Dense forest), movement towards South started only in the Iron-age after chopping down trees.
6) Even today it still got significant importance because most of the perineal rivers still flow in this region.
7)Most of the Hydroelectric power plants like Tehri, Bhakra Nangal, karcham wangtoo and Dehar dam located here. Indirectly these perineal rivers contribute to more than half of India's Electric power generation.
8)Many religions and Cultures flourish here because these plains are inhibited by different people through the history.
________________________________________________________________________________________________
Hope it helps you.
The reason is simple the people are there because there is a lot of food and agricultural land there so all the farmers are settled there. The credit
of this goes to the rich waters and soil of the region. Plus due to the ganga and Yamuna doab other occupation such as fishing, tourism ect are largely practiced .