When people feel uncertain - the cave in. People will move away from change and try to create some sense of stability in their minds and in their life.
Deep down they feel scared and really the only way to get them out of that state is to give them some type of certainty. Some type of belief, promise or descriptive the future of how things can and will be. Without that sense of direction no change can happen.
Marketing makes change happen, uncertainty is one of those things that stops it.