The most basic and fundamental reason is the very different exigencies of the people and governments of the players at hand. I will focus on the Chinese:
The treasure fleets did not increase imperial revenue. They were designed to awe and overwhelm other trading governments. The basic trade mechanism of this process was asymmetric imbalanced trade driven by the pouring of chinese goods into the hands of trade partners with no reasonable expectation of return (interestingly, trade deficits with China are now again the norm globally). After Zhu Di's death in 1424, resistance to these expensive expeditions significantly grew.
In short, they were profligate, a decidedly un-confucian quality.
For all of Chinese history, the majority of the economy was (and is) internal, with international trade representing only a small portion. So, when it became too expensive, they lopped it off.
The Grand Canal was rebuilt to an effective level by the 1420's, this made the need for coastal trade, and naval protections of the coastline, moot. So, naval ship production fell off, and trade returned to its previous, pre-Yuan, route - the canals.
For the iberians - the reason to increase trade down the W. African coast and around to the indian ocean was obvious: they were resource poor, the indian ocean was a source of infinite wealth.