But at the same time Hazrat Muhammad(SAW) became prophet also last 3 prophets before him(SAW) came, the people of India , Japan , China had bigger population and needed the guidance too because they were mushrik too on top of that they did not believe in Allah(SWT) at least people of Arabia believed Allah(SWT) although they were mushrik too. So people of Japan , China and India also in need of guidance at the same time .