Because his power was supposedly from drawing the life (and the Force) off those around him. Since the Exile was a wound in the Force, he couldn't draw power off him. Similarly, since Telos was a dead world, there was nothing but decay and death for Nihilus to draw off Telos, thus making him weaker than if he were in a place surrounded by life and the Force (ie. if he attacked at Korriban with the DS energy, or someplace like Coruscant, he would have been stronger).