Maybe the rarity of redheads draws me because I want my characters to be unusual and memorable. A favorite historical character of mine is Iceni queen Boudicca, who really gave the Romans what-for in 60 C.E. According to Roman historians Tacitus and Cassius Dio, Boudica was of royal descent. Dio describes her as "possessed of greater intelligence than often belongs to women." He also describes her as tall, with tawny hair hanging down to below her waist, a harsh voice and a piercing glare.
So if you have taken notice of my red-haired characters, you're not alone. I woke up this morning wondering why I choose to make them "carrot-pated," and my conclusion is, just to make them stand out in their rarity and specialness, and I hope that resonates with my readers by making them more memorable.