alt Hacker News> The plugs also prevent mixing voltage and amperage. The typical two vertical blades (5-15) are for 15 amp circuits. 20 amp circuits (5-20) have one horizontal + one vertical blade. The receptacle has a T shaped slot to match - that way you can plug a low-amp device into a high-amp circuit but not the reverse.
Alternatively you can just run everything at 230V and then you don't need a million different plugs as any wall socket can provide up to 3.5 kW, enough for any home appliance except for the most power hungry ovens and IH stoves.
Rewiring an entire country is a wee bit expensive. Even if the wires are rated to 300V (they usually are), transformers would have to be replaced, and they cost quite a bit. Also different sockets and circuit breakers, and a whole lot of billable hours by licensed electricians.
(But changing the voltage is easy compared to changing the mains frequency. Japan still has to live with 50Hz in one half of the country, and 60Hz in the other.)