The primary reason was that Zheng He's expeditions basically bankrupted the Ming Court.
Entire forests in Southern China (and part of Vietnam) were leveled to resource the timber needed for the massive ships of Zheng He's fleet. Ship building and maintenance proved to be very costly for China.
On top of it, Manchurian and Mongol Tribes renewed their raids into Northern China, and the Ming Court struggled to fund their army to repel the invaders.
hi! FYI, there have been a few related posts; check these out for previous responses: