The 18th Phantom would be 3 generations in the past of the usual "default" Phantom, which would be maybe somewhere between 1880 or 1950 depending on when the "present" of a given adaptation is. So that wouldn't be the place for a female one.
Besides, the idea of the Phantom, "The Ghost Who Walks," is that by passing on the mantle from father to son, he's created a reputation of immortality that makes him intimidating to evildoers and criminals (who are, as has been observed, a superstitious, cowardly lot). If he suddenly "became" a woman, that would kind of spoil the illusion.