Why did Gerald Gardner choose the Horned God and Moon Goddess as the divine patrons of his Revival Witchcraft and, by extension, of the entire Pagan Revival?
Well, in a sense, he didn't choose them: one could say—again, in a sense—that they chose him.
These, in fact, were two of the three available strands—the third being Ceremonial Magic—from which Gardner plaited the cord of the Modern Craft: the Murray/Horned God/solar calendar strand, and the Leland/Moon Goddess/lunar calendar strand.
But let us go deeper.
The Horned God, of course, is preeminently God of Animals and, as such, of the Body. Insofar as Revival Paganism personifies the Western world's necessary return to the body, its truths and cycles—the reembodiment of Western spiritual life—one could hardly choose a more fitting divine patron.
As for the Lady of the Moon: she herself is the goddess who grows, who wanes, who is no more—and who returns. Lady of Cycles, of birth, death, and rebirth, she in her very being shows forth the truth of the New Paganism. What once was, but was no more, is now again.
It was the flowing-together of these two currents—this confluence, this Great Rite of traditions—that brought forth the Modern Craft, and—by extension—Modern Paganism as a whole.
As for the agency of this astounding process—whether human, divine, or both together—I'll leave for you to divine for yourself.
Me, I'll take option C.
Above:
Confluence of the Minnesota (R) and Mississippi (L) Rivers