My theory is that the target fans have real world Oepedius (sp?) Complex. They really do have desires for their mom or aunty or older female relatives. By comparison, older men who buys AV don't have desires for their daughters or younger female relatives.
Older AV customers tends to fantasize about hot young girls in general, not just their own blood relatives, so they spread out their spending to all kinds of genre not just incest. Classic case is older gentlemen buy soapland or NTR AV because that's exactly what they do (visit soapland) or fantasize about (desiring the hot wives in their work or living environment). Younger AV customers (at least a significant segment of that) don't visit soapland, are afraid of girls their own age so they fantasize about the women who, not by choice, are present in their lives: mothers and teachers.