Power companies in Western Japan produce power at 60 Hz, while the generating firms in the East produce power at 50 Hz. This discrepancy has its roots in the modernization rush in the Meiji period. The link above gives a great capsule summary - the generating firms in Tokyo and Osaka imported equipment from different countries, and the nation grew to have two frequencies as firms in the East followed Tokyo's lead, and those in the West followed Osaka's.
While the frequency discrepancy doesn't cause problems for most devices that use mains power, the nature of power generation and transmission equipment is such that you can't simply send power from West to East, or the other way. There are a few conversion stations (three, to be precise), but there's only so much capacity per station. The combined 1 GW capacity falls far below the nearly 10 GW shortfall in the East. There is also some limited HVDC transmission capacity between the grids, though it isn't enough to transmit much more power.
The rush to modernize, and the uncoordinated growth stemming from long-held regional rivalries, leaves its mark on Japan even today.
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