Question

brahputra nadi ko banglades ma padma q bolte h

Answer 1

it's because bangladeshi people speak bengali and padma in bengali is another name for Brahmaputra

Answer 2

It is because Bangladeshi people speak Bengali and in padma is another name of Brahmaputra.
By the way, in Bangladesh Brahmaputra River is not called Padma, it is called Yamuna or Jamuna

hope it helps you.......................