Milk is not a simple liquid - it contains tiny globules of fat and casein which are not dissolved but suspended in water. When heated, fat being lighter than water, floats as a creamy layer on the top and water vapor, in the form of steam bubbles, is trapped under it. Further heating results in the formation of more number of bubble. These bubbles expand and lift the creamy layer causing it to overflow.
In the case of water, the steam bubbles break as they reach the surface. No film is formed and the bubbles of steam formed by boiling escape easily without any resistance. Hence, water does not overflow on boiling.