Low frequency was used simply because that was the maximum frequency that could be generated by the alternators of the day, higher would have resulted in them breaking from the forces in the rotor. They used a very large diameter rotor to get the magnetic field high enough to generate the required power, but so that eddy currents in the cast iron would be low. Stator used steel laminates, because that was where the AC was induced, though the iron also saw some of this field. As well the white metal plain bearings would not support a long life at high speed.
50/60Hz alternators ( aside from some very specialised ones for radio transmission) had to await the development of reliable ball bearings and the associated lubrication technology for the bearings, to become cheap enough and capable of handling the load. That you now have engines that use plain bearings and run at 12k RPM is due to increased metallurgy of the bearings, from the original wiped tin, Babbitt metal and lead bearings.