The majority of the population of those countries is distributed on a north-south axis. China's population is primarily on its eastern plains, not the western

The majority of the population of those countries is distributed on a north-south axis. China's population is primarily on its eastern plains, not the western mountainous regions. India has one time zone, but it's still covers more degrees of latitude than longitude, so the difference in sunrise time between Kolkata in the east and Bombay in the west is about an hour. This is about the same as the difference in sunrise time between Boston and Kalamazoo, also both in the same timezone.