The reason is because of the limitations of synchronous generators. We use synchronous generators (AKA turbo alternators) in thermal plants. The turbines of these turbo alternators have maximum efficiency at 3000 rpm to 3600 rpm. So when we calculate the frequency, (min of 2 poles) we get f= 50 or 60.
frequency, f = PNs / 120
The poles are suitably reduced to 2, because a large number of poles means, larger diameter of the rotor, which in turn means more centrifugal forces are developed during such high speed rotation.
Hence the power frequency is fixed at either 50 Hz or 60 Hz.