Frequent moves, inconsistent surroundings, changes in schools; all these things create instability in the environment. This is tough on children, and raises stress levels.
Children need to know they'll have a meal tonight and a warm place to sleep. They need to know they won't get shot if they play in their front yard. They need to know that there are others there to protect them, and that their fears and despair will be comforted.
A lack of security can affect a child in numerous ways. Play and exploration thrives in secure environments, it withers without them. Since play is important for the proper emotional and cognitive development of the child, insecurity can impact all aspects of child development. Insecurity in any one of these areas also creates paranoia and elevates stress. Not surprisingly, a secure environment has been postulated to be the most basic necessity for a child's development. Take away a child's sense of security and everything else withers in response.