Because it is not optimally structured, doesn't have an excess of ions or oxygen and doesn't hydrate as well as other water alchemies.
It won't hurt you per say, it is WAY healthier than tap water or mineral/spring water. What you DONT want in your water is chemicals (in all tap water or inorganic minerals (in most spring water.
Like I said its real good for cleanses, detoxes, fasts etc but not optimal for every day hydration or nutrition.