I seem to remember posting on the fragmented power grid much earlier in this thread. I have had to look it up again, I plead memory failure.
Eastern Japan including Tokyo is 50 hertz but the rest is on 60 cycle.
It dates from the 19th C.
Tokyo area selected German 50 hertz. Osaka and much of the west selected 60 cycle from the U.S.
There is a good article in Scientific American, but their website is down for revision.
Apparently there are only three small converter stations that bridge a fraction of the gap.
Yes, engineering could cure this, but a multiyear rebuild at billions of yen is involved.