Water is heavy. For spaceflight, what almost always counts is the performance of something per unit of weight/mass. It's not so efficient to launch a large mass of water that contributes nothing to the mission besides boiling off on reentry. Your article mentions this offhandedly: "But the weight of all that water on a real mission may well swell launch costs."

Water is a good coolant because it absorbs lots of heat energy in boiling. But that's not so efficient for its weight/mass, since a water molecule boils only once and then is gone. A solid material can disperse more heat per weight over a longer time without being consumed.