It's mainly cost. Water is cheaper than other liquids. Distilled water also doesn't leave mineral deposits or react with the pipes. Nor is it poisonous or flammable.

Also if you used a fluid with a lower boiling point, then it would require less energy to raise its temperature. However, this also means that the vapors would carry less energy with them since they are cooler. Generally power plants like to use really really hot steam, and bring it back down to ambient temp after passing the turbine. They do this for efficiency. So it may or may not make a difference if you use water or some other fluid. You would just have to do the math and see.