Because they wanted to continue trading/proselytizing in China and Japan. If they had continued to trade in slaves, the Chinese and Japanese governments would have shut them out. For instance, the first official Portuguese mission to China ended in disaster when the entire Portuguese delegation was executed by the Ming government. One thing that contributed to this hostility was that the Portuguese bought a lot of child slaves, leading to rumors that the Portuguese were kidnapping children in order to eat them.

The Portuguese did not follow the same policy in West Africa because the local authorities there were not as opposed to the slave trade. In fact, much of the West African slave trade was organized by local kings in collaboration with Europeans. The Kingdom of Dahomey was the most famous of such West African states. The King of Dahomey obtained most of his state revenue through the slave trade.

As mentioned by /u/zhemao , part of the reason for banning the sanctioned trade of Japanese and Chinese slaves is continued access to very prosperous markets for the Portuguese. Portuguese colonists, on the whole, were relatively unconcerned - most pressure came from outside forces. In 1571, King Sebastian of Portugal outlawed the trade of Japanese slaves at the behest of the Jesuits, as it directly impacted their missionary efforts in Japan (C.R. Boxer, "The Ship from Amacon", pg. 36 - published in 1959).

This didn't necessarily sit well for those already in possession of East Asian slaves, however. For example, in 1605, Phillip III was forced to repeal the 1571 decree under threat of a Portuguese uprising in Goa.

Macau also had problems controlling the trade of East Asian slaves, with Cantonese officials questioning the use of Japanese slaves over black slaves in 1613 and 1617. Author Tatiana Seijas describes the insistence of the Cantonese officials depicts a complex Portuguese Trade Network that was able to move people in many directions, operating just outside the law of many nations Tatiana Seijas, "Asian Slaves in Colonial Mexico: From Chinos to Indians, pg. 56.

The Portuguese Provincial Council of Goa listed the following 5 reasons the Portuguese could enslave peoples in 1567:

• 1.If their mothers were slaves (Womb law)
• 2.They were captured during warfare
• 3.If they sold themselves into slavery
• 4.They were sold by their fathers into slavers
• 5.They were committed to slavery based on crimes committed within their homeland.

Largely, these "just reasons" given were ignored, particularly in China, where the Portuguese colonists regularly took slaves from Canton to Macau. For example, the Jesuit Matteo Ricci is quoted as saying China was "virtually filled with slaves"... "many of them were taken out of the country as slaves by the Portuguese" (The Journals of Matthew Ricci 1583-1610. pg. 86. Random House 1953). Throughout the time described by Ricci, numerous decrees were issued by both the Portuguese and Cantonese governments (in 1595, 1613, 1619, and 1624), pointing to a continued problem with enforcement. (C.R. Boxer, "Fidalgos in the Far East, 1550-1770: Fact and Fancy in the History of Macao. pg. 235. The Hague, 1948).

An example of the continued trade can be seen in an account of VOC traders aboard Santa Catarina off Macau in 1603, which mentioned a cargo of 100 female slaves, likely destined for sale in the Philipines and elsewhere throughout SE Asia (Peter Borschberg, "The Seizure of the Sta. Catarina Revisited: The Portuguese Empire in Asia, VOC Politics and the Origins of the Dutch-Johor Alliance" Journal of Southeast Asian Studies, 33, No. 1, (2002), pg. 46).

Largely, it seems the opposition of the trading governments both in China and Japan, as well as complaints from proselytizing organizations due to slaving caused the Portuguese government to act. However, these decrees and rulings were difficult to enforce and still saw large numbers of slaves traded throughout the SE Asia region. I recommend browsing Tatiana Seijas, "Asian Slaves in Colonial Mexico: From Chinos to Indians, pg. 56 for some quick information, with more in the footnotes and elsewhere.

Please feel free to correct me if I am mistaken anywhere. It is late (midnight) and I may have missed something.