Is there a reason that multiple religions seemed to have coexisted better in Asian than in other parts of the world?

I'm currently researching new religious movements in Korea, circa 1860s, and I just finished reading a book that contains records from Christian missionaries going to Korea and declaring the place "without religion," despite the presence of several spiritual traditions. The exclusive characteristic of religion is really a Western concept - one can't be Christian and Buddhist, because that would negate the whole "thou shalt have no others gods before me" part of monotheistic religions. But this is a bit more fluid in Asian religions. A lot of the "religions" you mentioned are practiced more like "traditions," and there's no emphasis on proselytizing.

Today, in surveys on religious affiliation, many people in Asia would say that they are Catholic or secular or Muslim or whatever they are, but still participate in rituals on certain holidays or forms of ancestor worship or whatever has been happening in their family for generations. Anecdotal, but illustrative: my husband's family in Korea are Christian (his uncle is a pastor), but the whole family still celebrates Buddha's birthday, maintains ancestral graves, and donates to Buddhist shrines before big events (births, tests, job interviews). These "religious practices" are basically family and cultural traditions that don't negate their Christian faith at all. So although they mark "Christian" on a census, an outside observer would rightfully point out that most of them visit Buddhist shrines more than their Christian church.

As for some of the specific religions you mentioned... Confucianism is more of a code of ethics and social rules rather than a religion. It coexists easily with other traditions because it has no spiritual doctrine, really, except perhaps some aspects of ancestor worship. This is most simply demonstrated by the fact that the Chinese Communist Party has built "Confucian Institutes" to facilitate cultural learning abroad. Confucianism can coincide with secularism because it isn't really religion.

I wish I could tell you more about Shinto, but my knowledge of Japan is pretty much confined to the colonial period, so I really only know how it was applied in Korea post-1910. Shinto was state religion in Japan, and really closely tied with Japanese nationalism. From reading that I've done, I get the impression that Shinto was chosen to be the state religion because it was an indigenous Japanese practice rather than the number of followers it had. I know some Korean shamanistic rituals were redressed as Shintoism in the early twentieth century with little fuss, which goes to show that the rituals were the important part, and it didn't matter much what the associated religion was called.

With all of that said, there is still a long history of religious violence in Asia, although usually these conflicts were over political issues rather than doctrinal/religious ones. In Korea, there is a history of bloody repression of Catholics - although this was perpetrated by the state over perceived political subversion, and not religious conflict, per se. Buddhism, of course, comes to mind as well. The popular image of the "warrior monk" comes from the Buddhist tradition, and Buddhist temples historically have had a lot of power - some owned land, had armies, appointed officials, etc. More recently, Buddhist conflict with Muslim groups (in Burma, Sri Lanka, Thailand, etc.) are gaining attention, in particular due to suicide bombings and other terror attacks.

TLDR: Non-monotheistic religions play well with others. Religious violence in Asia tends to happen in conflict with the state rather than other religions, with recent exceptions.