Probably the first reason is pumping costs. Gravity is cheaper. Treatment of surface water to human health standards is probably more expensive that treating groundwater. They may also have not found groundwater in sufficient quantity.

They might be able to do what was done on the Elwha where they created what were called infiltration galleries next to the river. I think in the floodplain. I believe it was pumped from the galleries but it already had some filtering of the sediment at least.