Why ac current frequency is 50hz?

The generation of AC currents and relationship to frequency has to do with the speed of the unit that is turning the generator (prime mover).

The formula for frequency is f = Number of revolutions per minute of the engine (N) x Number of magnetic poles (P) / 120

Speed the prime mover up for a higher frequency, slow the prime mover down for a lower frequency. This is why the generator has to be wound for the voltage that it is going to produce and the frequency that it is to operate on.

This has probably got to do with Iron.

Most Transformers consist of large amounts of iron that need time in order to change polarity of a magnetic field.

Faster frequency would make the transformer less efficient. The iron would not manage to become fully magnetized. Slower frequency would make the transformer need more iron, hence becoming heavier. If the size of the iron core were the same and we just lowered the frequency, then as soon as the magnetic field is buildt up, then the rest of the energy until next half wave will be transferred into heat.

It is simply a trade off between efficiency and practicality.

Today's high speed transformers operate at speeds between 20-80 Khz and is only doable due to a special core material of powdered iron that is much easier to magnetize.

Electricity in the early days was DC only. However, some decisions were made to use AC because DC is, at high voltages, very dangerous to humans, something that was demonstrated during the first few 'victims' of the electric chair that in the beginning operated at DC current only.