Mostly, the lack of a Time Machine. In 1987, when I was creating the characters, I’d never come across the use of the singular they to signify a nonbinary pronoun, or even heard of it as a thing, and I was widely read and had a lot of trans and genderqueer friends. “It” was the word we had back then, so it was what I used. (I really like “they” for Desire, given that Desire isn’t gendered because Desire contains all possible genders, so would have used the word with delight if it had been around, or if it had crossed my path.)

(I had to fight mighty battles with copy editors just to be allowed to use the singular they in those long ago days. It wasn’t a concept that copy editors were familiar with, and I would have to wave an essay by Ursula LeGuin at them, which pointed out that the OED had uses of singular they going back to the 14th century.)