You have to keep in mind that trucking in water isn't feasible in a lot of places. Sure, in developed nations there is a good road infrastructure, but it's rarer the further out you go. And it's usually the further out you go that water is more needed. Then, even if you do have a road, have to consider the water truck breaking down, the driver not showing up, etc.

Even in developed nations, a water truck doesn't always work, for instance dirt roads with steep inclines and sharp turns.