Why mostly steam is used to rotate a turbine?

The Rankine cycle is one where a closed loop containing water/steam is used to transfer energy from an energy source such as a coal fired furnace or a nuclear reactor to a steam turbine/generator. It is the way all such power plants work, but not plants using gas turbines or combustion turbines which don't use a water/steam circuit.

In the Rankine cycle there are progressive stages: 1. Cool feedwater is pumped at high pressure into a boiler which is heated by the furnace or nuclear reactor 2. The water is heated and evaporated at high pressure, becoming steam 3. The steam is further heated to become superheated 4. The steam is admitted to the HP end of the steam turbine, and it progresses in stages to the LP end by which time it is cooled and under vacuum (the vacuum is maintained by external cooling water and increases the efficiency considerably by getting the maximum energy out of the steam) 5. The steam condenses in the turbine condenser under vacuum, to produce cool water, and hence back to stage 1. Electrical energy is extracted by the generator which is coupled to the shaft of the turbine, but that is not really part of the Rankine cycle, which is described in thermodynamic terms only. See link below for more reading and diagrams.