I think Zheng He's voyages were actually loss-making enterprises for the Chinese treasury, so the main reason that they were shutdown was that court bureaucrats slashed them to save money after the patron of the voyages, Zhu Di, died. The Confucian-trained mandarins at court looked down on the whole thing; it was expensive, designed to increase foreign trade (something they were skeptical of) and promised to increase Chinese involvement in far-off lands they saw no need to get entangled in. Of course, with hindsight, we know it was a bad decision, but at the time, and to the people making the decision, it was at least justifiable.