A big problem for the Japanese is that by the the 19th century, Russian colonization of Sakhalin had reached such a point that for a comparatively weak Japan, it was already quite difficult to colonize the southern half. I think there were agreements before the Russo-Japanese war to the effect that the Japanese would colonize only the southern half, and the Russians, the northern half, as a method of resolving conflicts between colonists. You'd have to have the Japanese reach Sakhalin before the Russians reach it, which would basically be before Russia is established in strength in the Far East.