Because as much as I like Steph I have zero interest in the book unless the old Batgirl is still a major player (of any kind).
I'm sure I'm not the only fan who felt this way about Cass. (Although I'm just as sure I'm in a distinct minority here on s_d.) And yet, she grew on me. Or rather her potential grew on me. Because obviously, DC doesn't know what to do with her.
And whatever editorial says, that is the reason she's being replaced (at least for now) by Steph. Cass's character is so far out from a lot of writers' experiences that they can't (for whatever reason) write her.