Why are turtle-doves emblematic of faithfulness in love?

Because of their adoption by the poets; - " as to its highly prized fidelity and chastity, setting aside idle fables, it presents nothing superior to other birds which lead the same mode of life." - Blumenbach.

The turtle-dove is also called the culver. Spenser, in a sonnet, has Like as the culver on the bared hough, Sits mourning for the absence of her mate.

The cooing of the turtle-dove of the United States sounds very melancholy, but is, nevertheless, joyful - this being, in reality, the notes of its amorous affection. The dove, in the wild state, breeds twice, but when domesticated, nine or ten times in the year; so that a single couple would, in four years, produce 14,672.