What is the number one reason that it projects fall behind schedule or fail?

Poor planning or poor project management is the number one reason why projects fall behind schedule or fail. We all know how important it is to plan before acting, even more in the area of management of the different areas of the project, it is important to define the objectives of the project from day one, defining them will mean success or failure of the project and from this point to advance in the planning of all areas before starting to do any work.