Board Thread:Questions and Answers/@comment-49.145.133.249-20170712050427/@comment-1458758-20170712104110

... Really, I mean why Frankenstein when Mordred "son" of King Arthur is right there?

Anyway, it's not a great answer, but I'll give it a shot. Back when Nasu first started writing Fate/Stay Night, he came to the conclusion that the female heroine would not be as popular as a male one, and yet, in trying to maintain the romantic relationship with King Arthur, decided to genderflip he into a she. Ever since then, some historic figures in the Fate franchise have been changed from Male to Female, seemingly just for the hell of it.