Synchronization condition: There are five conditions that must be met before the synchronization process takes place. The alternator must have equal line voltage, frequency, phase sequence, phase angle and waveform to that of the infinite bus/Grid. Because of this condition, we got 50Hz frequency. Assume having a grid with 50Hz and another with 60Hz, you can't synchronize these two and can't use an instrument designed for one in another unless you got some kind of converter. Because 50 Hz was decided a long time ago and transformers and instruments have been designed for that frequency. If the instruments were designed for another frequency then it wouldn't work properly and overheat (and get damaged). There are better frequencies but it would be too costly to pretty much rebuild the nations entire power grid (not to mention world's power grid). Every transformer in every pole, substation, and home appliance would have to be changed. 50 Hz Instruments works in 60 Hz mostly without any problem, where as 60 Hz Instrument may damage in 50 Hz (overheating of control transformer, fan etc.) Unless it is designed for 50/60Hz Although there are two, 60 Hz in the US and North America and 50 Hz in most of the rest of the world.