Perhaps because the pH of the water is rather trivial, especially compared to the acidity of an acid stop bath. And when I use the term "acidity", I'm referring to the amount of acidity that is available in the stop bath to neutralize the base in developer.
Originally Posted by r-brian
Whether your tap water is pH 6 of pH 8, it may have very little buffering capacity and have a negilgible effect on the neutralization on the developer other than merely diluting it. Of course, a pH 8 water can't "neutralize" the developer as it has no acidity.
Bill Troop recommends using a buffered stop bath for a high efficiency stop. It consists of acetic acid and sodium acetate combined to make a buffered solution which has moderate pH that is around 5, but it has a very large amount of acidity with which to rapidly neutralize the pH of the developer and stop development very quickly.