I'm calling BS.
WB isn't ignoring the opportunity they have there. They're essentially saving money by NOT having to make origin movies of The Flash and Green Arrow. It's all laid out for them. By having these two shows AND Batman vs. Superman, they're guaranteed to have:
Superman
Batman
Wonder Woman
The Flash
Green Arrow
I mean, whether you ignore 2011's Green Lantern or not, all they REALLY have to do is re-establish Green Lantern and they're all set. IT'S ALL THERE.
Personally, I didn't think Green Lantern was a masterpiece (far from it), but I accept Ryan Reynolds as Hal. That movie exists. Let it exist. His origin is there, he's established. Don't redo what you already did. You're already doing that with Batman.
BOOM - you've got your Justice League. And it was effortless.
Any other characters like Aquaman, Martian Manhunter, or Cyborg, it's not an emergency if we don't establish them right away. Put them in the movie, but it's not imperative to give them colossal amounts of screentime.