Costs to flush out and decontaminate the pipes would not be insurmountable. That along with different liquids have different demands on pipes, pressure, chemic

Costs to flush out and decontaminate the pipes would not be insurmountable. That along with different liquids have different demands on pipes, pressure, chemical reactions (corrosion) and then the viscosity factors. Maybe that pipes suited for oil are not suitable for water carrying, or not as ideal.