Why did Zheng He's voyages end?
The voyages were initiated by a group of eunuchs who had the Emperor's favor. They were opposed by the civil authorities. Zheng He was a eunuch.
The Civil authorities began to gain political power and the eunuchs began to lose favor after the death of the Yongle Emperor. The short period of Chinese exploration of the larger world soon came to an end. The Civil authorities by their nature would encourage government attention on internal matters.
Zheng He gained his reputation as a soldier for the soon to be Yongle Emporer. He would be considered an upper level civil servant for much of his life rather than a merchant.