why brahmaputra loses its name to ganges after their confluence when it is the larger river?

Answer 1

The source of the river that creates the confluence is Gangotri in the Greater Himalayas
As the Ganga flows from the North to the Bay of bengal in the East it is regarded as the holy & sacred river. Therefore the remaining portion of Brahmaputra after it merges with Ganga is also known as Ganga in India