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The reason is that this is in the optimum range for power supply on a national scale. The use of lower frequencies would cause the size, weight and cost of the installed equipment to increase and the flicker of lights to become noticeable (as it was on early 25Hz systems). The use of higher frequencies would cause increase in the operational losses due to eddy currents, hysteresis, skin effect, radiation etc. and reactive voltage drops.http://in.answers.yahoo.com/question/in ... 622AAnUVBJ
In physically smaller power systems such as those on planes, ships, submarines and even railways, higher frequencies are used because they allow reduction in the power equipment size and weight - i.e. the optimisation is different.
50 Hz however, is not universal. Many countries use 60 Hz which is close to the same optimum as 50. 60 Hz users mainly are: almost all the Americas and some countries in Asia. Korea even uses both frequencies! 50 Hz prevails in Europe and ex-British colonies.
Although David F fails to answer your "why" question, he does provide accurate information as far as it goes. Mike 1942's is not really an "engineering" answer.