Academics make reference only to other academics or to dead classics. It would be pleasant to think that this was an index of quality, and to a degree it is so; but not to a degree sufficient to explain the whole phenomenon. Some part, perhaps even the majority of this effect, is explained by a mixture of statistics and professional interest. The first of these is easily explained: Even if the academic were citing at random within the field, most of their references would be to other academics, since that is where the bulk of the accessible literature is produced. It is a matter of simple probabilities. The remainder of the effect, the exclusion of the non-academic writers other than the deceased, is is explained by the fact that such people are negligible as professional threats; as a career-constructing scholar, you have no reason to think that such people will ever be in a position either to assist or to harm you.