Because no one cares about Green Arrow, particularly now that he's been on Smallville for several years, thus spoiling any chance of novelty?What?
Why not just do a Green Arrow series instead? They already had Justin Hartley playing the character for several years now on Smallville.
If the CW does a DC Comics-based show, they'll need male interest, of the sort that Clark Kent naturally attracts but Ollie Whoever doesn't. Until they can convince WB to let them have a young Bruce Wayne, focusing on a bodacious heroine is a much safer bet in that regard.