Because there were no populated region in the east and therefore not much demand for goods.
Explanation:
Indus Valley Civilization was the early urban civilizations of the Indian subcontinent originating in 2300 BC which was established in the Sutlej, Ravi, and Indus river basins. They traded not only in agricultural products but also in colored gemstones such as turquoise, lapis and lazuli. Though jade was imported from China through sea route, the inland trade route to the east has not been found.
