I think because the notion of war crimes doesn’t mean much anymore. If there’s a crime, who is prosecuting it? Who has authority to bring criminals to court? The existing western led order and its institutions like the UN have lost legitimacy and power. So maybe they recognize it’s a war crime but feel labeling it this way doesn’t add much?

But at the same time I agree with the sentiment. It’s not the first time Iran has hinted that desalination plants are a target. Trying to control the strait is also a similar tactic - it cuts off food imports. And they also recently said they would target a nuclear power plant.

Did you read carefully? USA started it with an attack on Iranian desalination plants.