I suspect it's possible they plan a relaunch for many Batfamily comics (many of these comic sales are relatively low). So, maybe they're wainting for that time.
DC knows Batman sells well, so "Batman and the Outsaiders" sales are going to be good. However, the group has to be more profitable than other possible groups with Batman (or they can just put Batman in other group).
Black Lightning has a show, but this hardly will benefit the sales. Even successful show does not usually increase sales of a comic (since it is difficult for new fans to enter on comics).
PS: Is Duke the top selling male of color? I mean "Batman and the Signal" sold very well, but it was a book with Batman. So, I don't know how popular is Duke.