In that evil characters are inherently entirely "self-interested" and Heralds dedicate them to the "higher cause" of knowledge, at the expense of their own desires. Presumably, a necromancer who would be willing to give up his own desires to dedicate himself to knowledge wouldn't be a truly evil character, and would likely have to be turned neutral. In this event, the necromancer would likely forfeit his chances at liching. You can't both want to be the most unholy being on Thera AND be a noble guardian of knowledge. You have to pick one or the other. Thus, why evil aligns in general tend to be excluded from Herald, and especially necromancers.