Well, for one thing, it seems like you've got a rose-tinted view towards the acceptance and implementation of Buddhism in East Asia, and a few blinders on in terms of the acceptance of Christianity in the region. Simply put, Buddhism had centuries of commerce, interaction, and native Chinese/Japanese in high places who advocated for their religion. And even with all that going for them, there were plenty of mass purges of Buddhists in China. Less so in Japan, but the religion was often culled back in favor of state power over religion. Buddhism only became acceptable in China when the government became integrated with the Buddhist sangha. This seemed to begin with the Northern Wei Dynasty which, if I remember correctly, was Turkic in origin with a long history of contact and commerce with the Sogdian and Uyghur, many of which had transitioned from Manichaeism to Buddhism. After the Wei implemented many structural legitimacy rituals and economic incentives towards the Buddhist sangha, it became not only more accepted, but an integral part of the short lived Sui Dynasty and the much longer lived T'ang Dynasty. That said, the T'ang Dynasty was not immune to internal conflict, especially of the religious variety and the Great Anti-Buddhist Purge took place in the 840s. Though the reasons for it are complex and multivarious, and it's sometimes theorized that the Purge's main focus was on Manichaens, and it's worth pointing out in this thread that Nestorian Christians were also targeted.

Generally over the centuries, Buddhists in China after the T'ang Dynasty have been largely safe and put into positions of power. The Mongol Yuan Dynasty put Buddhists in high places and had a strong alliance with Tibetan Buddhists while the Ming Dynasty was founded by a former Buddhist monk, and the Manchu Qing Dynasty followed their Mongol predecessors and formed alliances with Buddhist powers both Tibetan and otherwise. When the Communists took over in the 1950s, however, Buddhism faced another round of persecution. Many monasteries and sacred relics were closed, destroyed, melted down, sold off, etc. Buddhism needed to change to be more "patriotic" to survive and Buddhism in China was transformed from a religious vehicle of government legitimacy to needing government approval in order to just exist at all.

In Japan, Buddhism was implemented from the top down. The Japanese Emperors and their families largely promoted Buddhism for the same reasons that Buddhism was promoted among China, Tibet, Korea and elsewhere. The Japanese royal family for a time probably also had an initial boost to spread Buddhism among the people: Japanese Buddhism is largely descendant of Baekje Buddhism, the smallest of Korea's Three Kingdoms. When Baekje was taken over by Silla, the Baekje Royal Family fled to Japan along with their court, taking religious relics, books, and monks with them, where they were safe.

Across the centuries of Japanese rise and fall of state control, the temples of Japan would expand in influence and economic power and start to flex their state muscles. This happened repeatedly, and the Emperors/Shoguns/Daimyo who always exerted control tended to deemphasize Buddhism to a significant degree. Oda Nobunaga, for example leaned more towards the Shinto tradition. Toyotomi Hideyoshi, his (more or less) successor, leaned back towards Buddhism. And (again, more or less) his successor Tokugawa Ieyasu took the country away from the Buddhist paradigm and shifted it towards a Confucian state bureaucracy.

On the other hand, it seems like you’re kind of answering your own question without looking at a broader context, which I’m happy to provide.

Yes Buddhism and Christianity are both “foreign” religions, and by their nature, religions that come from abroad are going to compete for the hearts and minds of locals which are most likely already with other religions. This is why Buddhism was not easily accepted by the Chinese and Japanese populations until they became ingrained in the government and why early penetration of Christianity into China (i.e. see the Nestorians above) failed to gain large swaths of followers. Nestorian Christians have a long and interesting history in China, but my knowledge basically starts and ends with the Nestorian conversion of some Mongol tribes, some of which ruled large swaths of China. (One of the most famous and complete early inscriptions of Phagspa Script we have is from a Nestorian grave, which is interesting enough given that Phagspa Script was invented to be able to easily convert Buddhist scriptures between Tibetan, Mongol, and Chinese.)

So while there was some penetration of Christianity into China (I can’t speak for Japan) up to 1500, it always had the tinge of foreignness, and more recently (in Mongol times as compared to Great Purge times centuries earlier) foreign imposition. Then a bunch of Europeans show up and really get the party started.

Greatly simplifying - I’m sure a few histories of China can tell you much more than this post will – there are two major periods that stand out when talking about Christianity as a global institution interfering in Chinese affairs. First is the Jesuits who came to the Ming Dynasty looking to convert the Emperor so they could gain access to China’s millions of potential converts, and second is the Opium Wars, the Open Door Policy, and everything that’s happened between then and the end of the Civil War in 1950.

The Society of Jesus, a.k.a. the Jesuits, were a missionary society who elected the strategy of learning the culture and language of the people before trying to convert them like some knock-off Jehovah’s Witnesses. In Tibet, Father Ippolito Desideri shaved his head, donned Buddhist robes, learned Tibetan, and learned Buddhist logic and debate format and was all set and prepared to debate the Buddhists in their own forum to convince them of the logic of Christianity when political events in Tibet forced him out. When he returned to Rome, he was ready to go back and start again, but dismayed by his dismal progress in converting Tibetans, the Church wouldn’t fund a second expedition for him.

In China, the Jesuits did the same thing: wore Confucian robes, learned Chinese, and compared their cartographical and astrological techniques with the Confucian scholars. Once they proved themselves to have equal, or ideally superior, knowledge as the Confucians, the Jesuits would then start promoting the connection between their knowledge and their religion. The Ming, being a state, had in mind the European technologies of war, which the Portuguese were eager to sell, with the Jesuits as intermediaries (greatly oversimplifying here). The Jesuits, in their negotiations with the Ming officials, were often happy to trade weapons for land (on which to build churches and missions). When it looked like the Manchu were going to take over China, many Jesuits didn’t “switch sides” but they did try to hedge their bets and started making overtures to get the Qing on board with Jesus. The Qing followed the old Mongol policy of basically pursuing every religion at once. They were historically close to Tibetan Lamas, especially of the Gelukpa and Kagyupa sects, pursued the Ming policy of Confucian bureaucracy, and later patronized the Jesuits as well. When the Fifth Dalai Lama went to meet the Shenzong Emperor in 1656, it was originally agreed that the Emperor would travel west towards Kokonor (modern Qinghai) and the Dalai Lama would meet him at a border town. It is alleged that the Jesuits feared having yet another religious competitor in the form of the Tibetan Buddhists and so convinced he Emperor that the stars portended danger, and that he should stay in China. The Emperor then invited the Dalai Lama to China instead, and he made the journey. It’s a tale with far reaching political implications which neither the Tibetan nor the Chinese records align exactly, or make much sense in the context of their own history. I’ll stop myself here because it’s only tangentially related to the matter at hand.

cont'd