Basically, it's a branding thing. I usually write hard science fiction and secondary-world fantasy. This time I wrote a cutesy halloween book with interior drawings. The overlap in those two audiences isn't very big. (Well, ok, it's bigger than, say, the overlap between historical romance and near-future military SF. Because even hard science fiction readers have kids.)
But on a broad scale, people who buy my Asimov's reprints aren't going to be the same people who fall in love with Arthur and Genevieve. If I publish them all under the same byline, they're going to show up all jumbled together anytime someone does a web search for "Nancy Fulda". In other words, it makes it harder for people to find the books they're actually looking for, and the last thing you want to do as an author is make it harder for people to buy your books.
So. Universe: Meet Sherrida Pope. She's me. Only cutesy.