Practical: As there are multiple very-incompatible voltages in use (110v, 220v), it's a good idea to not have the same plugs. Somebody will inevitably plug in the wrong device, and putting a 110v device on 220v can be pretty nasty. Dual voltage devices which are popular today (largely due to power semiconductors which make them inexpensive), didn't exist when plugs were designed!\n\nPolitical: Many ex-colonies of Britain were trying to shake off British influence as far as possible, which probably was a factor in picking different system for power device standards, among others.
How is it that different countries have come up with different styles of electrical socket pins?