Why 220@50hz and 120@60hz?

I suspect it was to do with how fast the old generators could spin before they flew apart.

...The City and South London system saw the first application of direct drive from the motors on to the wheel axles. [...] The current at 500 volts was supplied through a third rail of steel channel, supported on glass insulators from transverse wooden sleepers, and picked up by cast iron slippers. [...]

Other sections of electrified track appeared: Bow to Upminster in 1905, Paddington to Westbourne Park in 1906, Lancaster to Heysham in 1908. The London ones, influenced no doubt by the underground examples, kept to D.C., but for the Heysham line, single-phase 6.6 kV at 25 cycles, carried on an overhead trolley wire, was adopted.

Wide differences of opinion arose at to voltage and system, and from 1916, 1500 volts D.C. on an overhead wire became increasingly popular. [...] In 1956, the Transportation Commission decided on a 25 kV 50 cycles single-phase system as national standard. [...]

On the A.C. side, different practices developed in different countries. In 1930, for instance, the three-phase system disappeared finally from the Swiss Federal Railways, and was replaced by single-phase 15,000-volt current at a frequency of 16 2/3 cycles per second. About the same time, there was one scheme in the United States employing 11,000 volts at 25 cycles on the trolley line, with 300 h.p. A.C. motor, and another operating at 3,000 volts D.C. [...]

The Turbine Era
[...] Between the two world wars, the great reorganization of the electric supply industry in this country [Britain], which led to the construction of the 'grid', was formulated and carreid out. In 1917, a government committee recommended that all supply undertakings should be brought under one central authority, and in 1919 the Electricity Supply Act set up the Electrical Commissioners, who started work in 1920. Districts were organized under the title Joint Electricity Authority, but were often under suspicion from the undertakings and were not completely effective, so that in 1925, reconsideration of the situation by the Weir Committee led to the formation of the Central Electricity Board. Its main functions were the construction of a nation-wide transmission network, the adoption of selected generating stations, and the standardization of frequency. At the time, there were 17 different frequencies in use, and 80 undertakings operating on other than 50 cycles. [continued]

The other, more 'Urban Legend' reason why the US choose 60Hz was in one of its first applications--The Electric Chair--it was determined that 60Hz was the optimum Frequency to Kill a human. This then became the standard. LIKE I SAID I HEARD THAT RECENTLY FROM AN MIT PHD Professor. Doesn't make that much sense though if you consider that same danger is being carried out to every home.