As electrons from the cathode gain more energy, they tend to ionize, rather than excite atoms.  Excited atoms quickly fall back to ground level emitting light, however, when atoms are ionized, the opposite charges are separated, and do not immediately recombine.  This results in more ions and electrons, but no light.  This region is sometimes called Crookes dark space, and sometimes referred to as the cathode fall, because the largest voltage drop in the tube occurs in this region.