"Is it because India is one of the few places on the world map that is poorer than China? "
Hell no, there are plenty of worse sh1tholes, for example there is Outer Mongolia which exports prostitutes into north China and then there is Vietnam which exports foreign brides into south China. But you see, I am a magnanimous man and not inclined to mock our inferior cousins overly much.
If you thought a wealthy and prosperous country like Japan, women would have little need to sell themselves for sex, then you would be right, those filthy Japanese sluts offer themselves for free.