Why was the route from India to the Persian Gulf traveled by seafarers much earlier than the route from India to China?

On the shores of the Persian Gulf in the 4th millennium BC. and the first civilizations of the Earth – Sumerian and Elamite – were formed. Their sailors sailed east as early as the 3rd millennium BC. e. and contributed to the construction of the first cities in Western India (in the Indus Valley). A millennium later, the Indians themselves settled Ceylon and began to explore the shores of Indonesia, but they reached the shores of China only in the last centuries BC.