Until everyone does it and the pricing structure changes.
People tend to have a lot of misconceptions about the energy industry. End-suppliers, like your utility company, don't care about the absolute price at which they sell* - They hedge their entire supply, and charge a more-or-less fixed margin on top of that. They don't make more or less when the actual supply price goes up or down.
Now, on the supply side, you have multiple layers of demand loading. You have the dirt-cheap baseline load, then you have some spare capacity in the baseline systems (usually not that much, on the order of 20-25%), then you have easy-to-online spare capacity (often from some form of short-term storage), and finally you have peak load generation (normally-offline gas turbines, in general). Of course in reality you have far more tiers than that, but you get the idea.
All of those cost money to maintain, whether they feed the baseline or the peak. The peak costs more because you essentially have generation capacity sitting idle all but 5% of the time, yet it needs to pay 100% of the bills to own and maintain it. So even there you don't have the suppliers making out like bandits.
Disclaimer - I can only speak for the US market. They may have something radically different in Japan, but overall, I would bet my eyeteeth they still use some form of hedging to maintain relatively fixed margins.
* While technically true, price does tend to affect how much effort end-users put into conservation. So believe it or not, your utility would rather offer you cheaper power, because they make the same margin per KWH whether you use all LED lighting or you heat your patio with a hair-dryer running 24/7.