Wood

It remains uncertain why the Ming Emperor stopped the construction of new ships after the apparently successful voyages of Zheng He (鄭和) - it seemed unlikely that the young dictator had sufficient foresight to predict communication with foreign countries would shake his reign.

There are several hypothesis. The one that I favour boils down to the fact that the capital of the empire moved from Nanjing (應天) to Beijing (順天) around the same time - with the inevitable happening that a new palace was needed.

Yes, it was the Forbidden City (紫禁城); and you also need all other buildings related to it, such as the Temple of Heaven (天壇), the Ancestral Temple (太廟), and so forth.

You know what: this new palace was made of wood. Specifically, most of it was Phoebe zhennan (楠木) - huge trees from the southern part of China, which took a thousand years to grow, and with the trunks that were resistant to insects and fungus. The process of transportation of those woods - from the forest of Guangxi (廣西) to Beijing - took almost a year.

The very fact was: this gigantic project possibly exhausted all suitable wood in the country, leaving very little for ship-building. No, it is not a problem of manpower. Workers could easily turn their job to make more ships once the palace was done - but it took some hundreds of years before the supply of wood was back.

Those were the years of Renaissance and great scientific advances.

PS. Once you learn how they named their new capital, you can say for sure that the Ming people would not prosper: they didn't try to make appropriate response, but would simply accept what came about.