Why did China stop sending its giant treasure fleets across the Indian Ocean?

Nobody knows for certain, and that is the honest answer. Between 1405 and 1433 Ming China's treasure fleet undertook seven maritime expeditions, led by Admiral Zheng He, and then the voyages ceased. It is not exactly known why they completely ended in 1433.

The fleets were big. The Mingshi records 62 treasure ships and a crew of 27,800 for the first voyage, though historians still debate the size of the ships and the fleet. The first three voyages reached Calicut on India's Malabar Coast, the fourth went as far as Hormuz, and the last three reached the Arabian Peninsula and East Africa.

One explanation is cost, but it is disputed. Some historians suggest that heavy costs partly contributed to the end, but others note that the costs had not overburdened the Ming treasury, and one argues the voyages were profitable. A second traditional explanation blames Confucian conservatism.

A third, more political, explanation looks at who ran the voyages. They were commanded and overseen by the eunuch establishment, whose influence depended on imperial favour, while civil officials were the eunuchs' primary political opponents and the faction against the expeditions. Another historian argues those officials were more motivated by institutional self-interest than by ideology. The Yongle Emperor also turned his attention and funds to campaigns against the Mongols, and after the capital moved to Beijing the seas received less and less attention.

So several explanations are on the table: money, factional rivalry, Confucian ideas and a shift of attention to the northern frontier.