Unable to? The same Yoruichi who clearly shows she can go to and from Soul Society whenever she damn well pleases?
Let's stretch disbelief a little more. :no:
In Soi Fong's case I can see her leaving Naruto behind after he's born, since pre-reconciliation with Yoruichi she's clearly a wee bit obsessed. But I see no such excuse beyond a lame Deus Ex Machina for Yoruichi to do the same.
Furthermore, having Yoruichi as the mother makes the fic lighthearted... and utterly predictable. Yoruichi is TOO MUCH like Naruto for her to hold any interest that way. She'd mess around much like he does, and probably treat him the same as she did Ichigo, both when she's relaxing and when she's serious.
Soi Fong, on the other hand, offers an interesting contrast. You're assuming that the kid HAS to be like the mom, but when the mom has been ABSENT for the kid's whole life, their personalities being exact opposites isn't rare at all, so the personality issue is worth squat. A parent can only shape the kid like him or herself if s/he is PRESENT to do so, and unlike what you may think, personality is NOT genetic.
My dad, whom I never met in person because my mom essentially scared him away in a pregnancy mood swing bout, was, according to my grandma, a serious man, if weak willed. She says I'm his exact opposite. So much for Soi Fong's personality being an obstacle.
And arguing her personality as being the reason she wouldn't get knocked up in the first place is a poor argument as well. All one has to do (and SR implied it in earlier notes) is to have Soi Fong be interested in Arashi's techniques, an him suckering her into a few dates in exchange for coaching.
With enough interaction, and assuming Arashi isn't a total klutz with women, I can see her becoming at least marginally interested in the man as she hangs around him, enough to even return during successive vacation periods, say once each year, for more training, and eventually sleeping together, just this once.
Or, if Shinigami vacations indeed last afew years, I can see her hanging around directly, initially just to learn techniques from him, then to learn about him... and most of the rare stuff we learned about Arashi points to him as a strong, determined man... the type of person Soi Fong can grow to respect, and eventually like.
Yes, it'd require at least a couple chapters of flashback mode, mayve even three, but it can be done in that amount, and then it's back to the story. The Kakashi gaiden worked that way, and nobody disliked it, which is rare in itself for Naruto, so why not do the same for this fic?
I rest my case.