My theory on it:
In order to be successful at magick, you must awaken your energy centers, in order to be able to receive and transmit the energies necessary to perform magick. These energy centers are commonly called chakras. And the chakras most essential to psychic development - the heart, third eye, throat, crown, etc. chakras - are characteristically light, not dark. Dark "magickians" certainly do not have enlightened crown or heart chakras, which prevents them from performing great magick like those whose hearts and crowns are awakened (Buddha, Jesus, etc.)
This is why saints and deities - people of psychic power - are often portrayed with illuminated heads (halos) and illuminated hearts. Those chakras are activated in them.
However, dark magickians do not have those centers activated and are rendered ineffective.
See the symbolism of illuminated heart and crown in this painting? That is part of Jesus's power.
That is just my idea on it.
a woman i knew once told me, that there are different ways to work through the seven central chakras; she told me especially of two paths:
the magical path builds its way up from the base, 1,2, 3 and so on.
the mystical path of working through the chakras is actually 4, 7, 1, 3, 5, 6, 2 (which is, so i heard, also the way in which the rosecrucians light the candles on their sevenfold candlesticks)
so in the mystical path you start with the heart. you jump around quite a bit, and seen from the outside there doesn't seem to be much development happening for quite a long time...
in the magical path, you build from the root, sexual power, willpower - that's chakras 1,2,3. and since you're very powerful then you might perhaps end up thinking that's good enough, and not proceed to the heart. that's the danger of the magical path
that's approximately what she told me.
...so perhaps, if one would like to be a real black magician, one should build up the power in chakras 1, 2, 3, without ever intending to go any further...