In a sense, China always had more issues to deal with on land than at sea. Ming China (the core bits anyway) was approximately the size of half of Europe and several times its population: internal and external enemies were therefore proportionally much larger than what Portugal, France, or any other European power had to deal with.
This is not to mention the resurgence of the Mongols in the late 1400s under Esen Khan, the emergence of a unified Japanese state following the Sengoku period in the 1500s and the Imjin War, and finally the renewed threat of the Manchus in the 1600s. The Ming state spent disproportionate effort manning the northern frontier; there wasn't much left for an official push into the south - and one could argue that without state shouldering of risk, mercantile interests had no incentive to mount expeditions into the unknown.
The Ming dynasty itself was also a 'national-security state' of sorts, very suspicious about internal threats to its rule and frequently employing an array of agencies to control dissent. This also meant that it sought to control population movement, a tendency that only increased when the arrival of Japanese wokou created large refugee populations on the coast that were as likely to become bandits and pirates as they were to submit to Ming rule. A series of 'Coastal Ban Edicts' would be issued throughout the course of the mid-Ming, which banned all oceangoing activities and created a 'zone of exclusion' stretching c.10 miles from the coast. This method was again used during Qing times, and unsurprisingly it really tanked Chinese naval aspirations.
Generally speaking, the bulk of Chinese navigation was riverine, rather than sea-based. While in Europe international mercantile transport largely required mastery of the sea due to geographic reasons, in China you had the Yangtze and the Yellow River serving as two major conduits linking the country together. With the construction of the Grand Canal during the Sui, the merchants in the prosperous Yangtze Delta also gained the ability to transport goods to northern cities without going through the risks of shipping their goods using a sea-route. Not to say that oceanic activity never happened, but it simply wasn't such a big deal for China as it was for other nations.
