Well, they had Man of Steel and Wonder Woman and Batman v Superman. The only one missing was the Flash. I guess Aquaman came out later. No one wanted to see a Cyborg movie.
I think if BVS was instead a 'World's Finest' movie that doesn't friggin kill Superman and maybe also release a Flash movie (that was lower budget like Shazam), they could had done alright.
Or just hold off on Aquaman and use someone else like Green Lantern. For better or worse, the Ryan Reynolds movie did introduce audiences to the concept behind GL.
The studio obviously aren't averse to the idea of Green Lantern in itself. John Stewart was a major part of the DCAU JL/JLU shows, Hal Jordan featured in his own animated series, there have been three animated Green Lantern movies (four if you count Justice League vs the Fatal Five, which is really a Jessica Cruz story), the 2011 live-action movie, appearances in various JL animated movies, and the upcoming HBO live-action series (which is now going to focus on John Stewart - obviously why they made him the central character of the recent Beware My Power movie). They seem determined to keep the idea out there.
And as regards Harley, I'm surprised she hasn't actually had a solo animated movie yet. I know she's got the TV show, but I would have thought they'd have given her her own animated feature. Closest she's got to headlining her own was the Batman and Harley Quinn movie a few years back.