Russia did colonise the west coast. It's now called Alaska.

In Japan, the Japanese instituted a policy of "Sakoku," which prohibited any foreigners from landing or any Japanese from leaving, in 1635, which lasted until the 1800s. As a result, nobody explored anywhere. A few Japanese Christian converts did move to Mexico, but that was about it.

In China, around this time, the Ming dynasty was collapsing, and the Qing Dynasty was too preoccupied with destroying the Ming holdouts. The last thing on their mind was overseas expansion.

Japan had a strict policy of isolation. Foreigners were prohibited from entering the country (on pain of death) except for a small trading island, and Japanese citizens were prohibited on similar terms from leaving the country.

I'm not sure about China specifically, but I don't think they had much interest in foreign affairs. There was plenty of internal conflict to clean up. Korea was poorly organized and in no place to sponsor overseas anything.

Russia however did set up several outposts on the west coast of North America. These lasted from roughly 1780 to 1840, and were small and abortive affairs. There was no real effort to establish a living colony, (Russia had all the land in the world, what did it need permanent settlements for?), and were instead strictly focused on trade and resource acquisition. Furs and metals from Alaska and northern California were desired.

Very interesting to note though is that this Russian expansion is in large part responsible for the Spanish colonization boom of California. Hearing that the Russians were encroaching on their turf in North America, Spain went to great lengths to grab land and establish villages/missions/fortresses along the coast of California, convinced that the Russians were nearby. In reality these outposts were thousands of miles away, and the Russian settlement in California was comprised of something like 100 people total. Not knowing that the Russians were so far away though, Spain hurriedly pushed for more expansion.

I can give an answer for China at least. The Chinese dynasties that ruled during the relevant period were overwhelmingly preoccupied with Central and North-East Asia - the semi-nomadic tribes that inhabited those areas such as the Mongols, Jurchen and the Manchus were a constant threat to the Ming dynasty (1368–1644), which was eventually overthrown when the Northern tribes unified under the Manchus. This was the time when most of the Great Wall as we know it today was constructed - to keep such invaders out. Under these conditions, colonial acquisitions were simply not on the cards.

The Qing dynasty (1644-1911) that replaced the Ming was made up of Manchu nomads, faced less of a threat from the North, and did in fact feel secure enough to engage in colonial expansionism, pushing China's borders westwards to incorporate Xinjiang and Tibet. I suspect the three main reasons why they never tried for the Americas were: 1 - the Manchu ruling families were steppes nomads and had no experience of the seas or of naval management; 2 - Manchu rule over China was secure against rebellion only because of a network of military garrisons, and the dynasty was unwilling to divert resources or attention away from them; 3 - it never occurred to them to do so - their Central Asia heritage, combined with strategic concerns inherited from the Ming, led them to focus on expanding westward rather than overseas.

Sorry, but your submission has been removed because we don't allow hypothetical questions. This sort of thing is better suited for /r/historicalwhatif.