Taller runners are less efficient because as height increases linearly, weight increases exponentially. To demonstrate this, think about how much weight a baby gains if it grows one inch. Now think of how much weight Shaq would gain if he grew one inch.
You should also add that it's not long legs by themselves that make people faster, that it's long legs relative to total height. This is one of the reasons those of african descent dominate T&F: they have longer limbs relative to body height than us whities.