Why some countries are using electrical frequency of 50 hz and the other countries are using 60 hz?

The choice of grid frequency is an arbitrary one. There are no particular advantages of one over  the other as those frequencies are quite similar from an electrical perspective.But if you consider the design perspective then there is a difference between these two. For Hz operation prime mover is to be rotated at a faster speed when compared to Hz. This implies that turbine design is to be made such that it gives high efficiency at higher speeds. (A similar phenomenon is prevalent in all power system components which require individual analysis).The skin effect is more predominant in Hz system. Another thing that is worth mentioning is the power quality associated with frequency. The higher the frequency, the easier it is to eliminate harmonics using filters.P.S: The design of power system components and loads is optimised according to the operating frequency. Hence, I think  there is no distinctive advantage of one over the other.