I don't know the real reason why they did this, but the first thing which came up to my mind was that in some cultures they use water to wash than to wipe, and

I don't know the real reason why they did this, but the first thing which came up to my mind was that in some cultures they use water to wash than to wipe, and that results in some unsanitary conditions from the perspective of paper users. There are technological solutions for that (like a bidet installation), but chances are some one low in the command chain made that decision. Also decision was probably 'If you wanna use water, use that bathroom', and it resulted in this effect that Hispanics used one restroom and whites/others the other.