Despite the West of Japan having a power surplus it can’t help the East, which is short if power thanks to the post-tsunami nuclear power problems.
Incredibly Japan is split by a 50 Hz/60 Hz incompatibility. The West based its power generation on 60 Hz American standards while the East uses ‘European’ 50 Hz. There are few interconnects and they can handle little power.
Neither choice explain Japan’s unique decision to standardize on 100 volt power rather than US 110 or EU 240 volts.