Overhead and initial cost of installation. Desal is still a WIP, we looked into it here a couple years ago and built one that was a total failure. The water was potable but no where near are normal aquifer water.This was a public water supply in the Caribbean. Actually a couple of different islands. But they aren't exactly known for their maintenance on things, so who knows. Water always tasked salty in some places. Never really satisfying.
Which also makes me wonder why they burn diesel fuel to produce electricity when they are surrounded by an ocean of waves and a sky full of wind and sun.