Water injection


I picked up Techs Aquamist water injection system and that leads me to a few questions. From what I understand the water is activated via a boost/pressure signal to a Hobbs switch. OK that is a simple system as it is the same thing I use on my nitrous on the fuel system side. What I don't agree with or rather quite understand is why is it done via boost/pressure as opposed to temperature? When you hit X amount of boost and the water turns on it is spraying a system that is relatively cool yet. In a sense it is costing power at that point. Why not have it come on at a temperature setting regardless of boost or pressure? Just curious?
Got it, thank you.