Ballistic missiles give of bright, very hot exhaust plumes on leaving their silos. Any major world power with a down looking infrared satellite (or watching for

Ballistic missiles give of bright, very hot exhaust plumes on leaving their silos. Any major world power with a down looking infrared satellite (or watching for "WTF was that launch in my wheat field" tweets from farmers"), will know when it launches, and have a relative idea of where it's headed and when it will land. A hyper-sonic cruise missile is much more stealthy, giving it the element of surprise.