Because the same kind of economic logic did not exist in China as it did early modern Europe. Zheng He's voyages was primarily a prestige project: it was meant to show the flag and display Chinese power and influence in the Indian Ocean and beyond. That means they were not economically self-sustaining: the voyages did not generate enough revenue to justify them and thus the program gets put on the back-burner or axed when there is a revenue shortfall or when the budget need to go somewhere else: what killed further oceanic voyages were increasing military pressure brought on the Ming dynasty by renewed Mongol attacks from the north, which meant the budget for more voyages no longer existed because money was needed to build armies and strengthen the great wall for defense purposes.

European voyages OTOH were easy to justify to merchants and governments because there were trade goods in the far east which sold for high prices in Europe: spices being the case in point. A single successful voyage of a ship to Malacca straits and return loaded with spices is enough to make the captain rich. And European governments of the era were desperately seeking additional revenue to fight wars against their neighbors which meant they were far more likely be patrons of such voyages.

There was also the dynamic of inter-state competition: much of the colonial ventures of the French and English for instance was in direct reaction to the Spanish conquest of Mexican and Incan gold mines which were funding Hapsburg armies in European wars. Hence the enemies of the Hapsburg were exploring North America for either their own goldmines or some kind of a profitable trade route towards Asia.

In China there was no distant trade goods which sold for exorbitant profit margins, and the width of the Pacific was such that it was not practical for China to engage to trans-oceanic colonial ventures, in Europe the economic logic to make oceanic voyages to generate profit was there, in China a mixture of geography and geopolitics logic meant that wasn't any sense to make distant voyages.

/u/RealMadrid001

Poner una pica en flandes?

When it comes to the alleged sizes of Zheng He's ships, you really should read prior posts on this.

1, 2, 3. This is asked fairly often and to be honest, most are tired of asking the same question again and again.

Now, going to comparing the motivation aspects (aka economics), /u/Desoulis pointed out some interesting facts, but I want to add more strongly that Zheng He's ships were really not intended, nor made feasible for, trans-ocean voyages.

Whereas in the Iberian peninsula, the Portuguese had sponsored decades and centuries' worth of exploration down Africa's west coast. Castile y Leon, sensing opportunity and desperation, sponsored Columbus' voyage even if their experts knew it was based off erraneous assumptions about navigation.

When it comes to Magellan, people often forget that he was originally a Portuguese explorer who joined Almeida as Almeida took his position as viceroy of Portuguese India. Returning to Portugal, he fell out of favor of the court and came to the Castilian court instead. The Castilians were interested in further exploiting the Treaty of Tordesillas, and arguably they were successful in doing so, as they could claim the Philippines against the Portuguese in SE Asia.