It's extremely cold? Vladivostok is Russia's main city there, and only kept ice free with the help of ice breakers and a thermal plant.

https://travel.stackexchange.com/questions/79051/is-the-port...

If memory serves, just a bit further south the ports are ice free. Russia had a port in Lüshunkou (then called Port Arthur). It is just west of Korea, and without ice. It was the subject of the Russo-Japanese war.

While looking that up I also saw that the Ming dynasty banned Han chinese from settling in the north for 300 years. So, that likely also had an impact.