Why is the electrical frequency is 60 hertz or 50 hertz?

The frequency of 50 or 60 Hertz (cycles per second) was selected to reduce inductive power losses caused by long distance power line transmission.

These fairly low frequencies still enable alternating current to be converted to various voltages and currents using moderate sized Transformers at the point of power use.

The higher the power frequency the higher the inductive losses over a length of power transmission line.

For this reason frequencies higher than 60 Hertz are avoided.

Where possible electrical power transmission over long distances these days is accomplished with high voltage direct current, which is converted to alternating current near the usage areas.