Political instability: The fourth century saw the collapse of the Western Jin Dynasty and the beginning of the Sixteen Kingdoms period, a time of political fragmentation and warfare. This made it difficult for merchants to travel safely and trade between different regions.
Economic decline: The prolonged period of warfare and political instability led to a decline in economic activity and a disruption of trade routes. This made it difficult for merchants to find buyers for their goods and to make a profit.
Population decline: The warfare and economic decline of the fourth and fifth centuries also led to a decline in population, which further reduced the demand for goods and services. This made it difficult for merchants to find a large enough market for their products.
Natural disasters: The fourth and fifth centuries also saw a number of natural disasters, such as floods, droughts, and earthquakes, which further disrupted trade and made it difficult for merchants to travel.
Rise of Buddhism: The spread of Buddhism in China during this period also contributed to the decline of long distance trade. Buddhism emphasizes a simple lifestyle and discourages material possessions, which led to a decrease in the demand for luxury goods and other items that were often traded long distances.
These factors combined to create a difficult environment for long distance trade in China between the fourth and sixth centuries, leading to its decline.