Japan is of course ethnically, culturally and linguistically homogeneus, an archipelago with few geographial features that could act to divide different states, and no state in history has ever want to be separated until the Ezo Republic, which was more or less the first state to not to want to be THE government of Japan. Powers in the archipelago have always wanted to BE JAPAN rather than being apart. They always recognized the same religion (more or less, it's more complicated than that) and the Emperor, and they always saw themselves as one people. Warlords acting independently is not even balkanization, as that was also the case before the Sengoku Period and to a degree, after during the Edo period.
Well, Japan remained feudal quite late which kept it unified since the daimyos were in many respects rulers of their own independent little realms under the shogunate. If any became too powerful they would get squashed, usually IIRC by a federation of other daimyo acting as the shogunate armed forces.
Hokkaido however was of a different ethnicity originally, IIRC, which lasted longer and still lingers in part. Only its relative remoteness from anywhere else with a population means that in TLs its almost bound to end up Japanese
Best Regards
Grey Wolf