I asked a similar question before and received no real answer (just links to other questions from the past that did not directly answer this question). So I'm trying again.
I've read about Chinese/Japanese explorers being quite apt, their cartography being fairly sophisticated as well. It seems to me that the west coast of North America would be "easier" to discover and explore than the east coast. Given that the Bering Strait is so narrow that on a clear day I've heard you can see Alaska from the Russian coast...whereas the "easiest" way to discover the east coast was hopping from Iceland > Greenland > NA, which is a pretty signficant journey compared to say Hokkaido > Alaska. What's more, there have been villages/groups of people on mainland Alaska for 10,000+ years so there would have no doubt been trade opportunities.
How far did the Asian powers explore to their east? What stopped them from discovering/exploring/colonizing the west coast well before 1500?
There's some evidence of at least hand-to-hand trading from China/Japan to Alaska in the pre-1645 era. Archaeological digs have uncovered cast bronze items that could not have been manufactured in Alaska or Siberia before Russian expansion into the region. The Koryak people of Kamchatka are known to have used steel weapons before Russian contact, but there is limited evidence of such weapons in Chukchi areas and none in Alaska.
As /u/786888786888786 states, Chinese cartography doesn't reveal the presence of Alaska, and I can state that Japanese cartography doesn't, either. As late as 1802, there were not even definitive maps showing Sakhalin Island. Kondo Morisge's Ezochi Zushiki is vague on the whole thing, and it's one of the more famous maps of northern Japan in that period.
Here's a nice overview of Japanese cartography by Kazutaka Unno.
For China? Frankly they thought Japan was to the east of them. There wasn't much incentive to explore any further east of Japan because they never had any intention of conquering Japan in the first place. The last time a Chinese dynasty conquered east was during Wu Zetian's failed campaigns in Korea during the 600s, roughly around 1300 years ago. For more than a thousand years, China's capital was located to the west in landlocked modern Xi'an, so imperial funding for explorations most likely didn't happen. It's unlikely that Chinese nobles ever envisioned their realm as a seafaring empire, even if they were ever more disposed to it as their traditional western and northern homelands were taken from them through nomadic incursions. The closer coastal capital of Beijing that we know today has its origins in the Mongol invasions when Kublai Khan moved the capital to Dadu, and in the Ming Dynasty it was permanently relocated by Yongle not only because that was his original fiefdom, but also as a pre-emptive move against further nomadic incursions. To give you an idea of Chinese conceptions of the entire world during the Ming Dynasty, here is the earliest surviving Chinese world map, dated to 1389.
I've read about Chinese/Japanese explorers being quite apt, their cartography being fairly sophisticated as well.
Do you have any source for this that I could read? The only source covering Chinese cartography that I know of is History of Cartography: Volume Two, Book Two - cartography in the traditional east and southeast asian societies, but I haven't gotten around to reading it.