They are thickly populated mainly because
1)they are holy places of India.
2)they are very famous because of old Hindu legends.

Hope it helps you :)
The Ganga and the Brahmaputra are thickly populated because : 
1. They are the most scared and the holy place of our country India than any other place.
2. These places are situated in all the delta places which are the highest one who have a good fertile soil. So, we can use it for cultivation hence it is thickly populated.
3. The ganga and the Brahmaputra are the rivers flowing there it self so the water supply is in more amount hence these places are thickly populated every time.