Why is water potential of pure water zero?

Water potential is how much water 'wants to move' - i. e. if something is very concentrated and something else is less concentrated, the less concentrated water will have a high potential so it evens out the concentrations.

Pure water is just water, so unless water with impurities was on the other side of a partially permeable membrane, there's no reason for the water to move anywhere hence it has a low water potential.