Why do the red blood cells have the longest life span?

Actually red blood cells do not have the longest life span of circulating blood cells - that would be differentiated memory B lymphocytes. RBCs actually have a relatively short circulating life span (about 4 months on average) because they lose their nuclei and most cellular organelles when they mature. This means the cell cannot repair damage to the cell membrane and eventually the cell becomes too damaged to function and is removed by the spleen.