It could also be because Tynion doesn't want to use the characters in his story (he wanted to use his own new characters). Batman would still be the better option to promote a new book like Batgirls, since the sales are still bigger.
That said, the backups in Batman comics will have a Batgirls story. So, the promotion is still there.
Now, Tom King has mentioned he wanted to be part of the crossover. And it wouldn't be so weird he really wants to show so many characters.