Board Thread:News and Announcements/@comment-807326-20161221093022/@comment-1075764-20161225002002

Hambergite wrote: Part of the reason that they didn't include Supergirl in the same universe is because they weren't sure if the show was going to be successful.

Maybe, but then also consider the other details:
 * One has to suspend disbelief already that Barry doesn't help out more in Star City since he has the power to stop basic crimes with ease. Putting Superman and Supergirl on their own Earth lessens that kind of problem.


 * Plus Supergirl actually did introduce a continuity problem, well if it was to be set on Earth-1. Oliver was pretty much established as the first modern costumed hero of Earth-1 (the JSA predating him of course as shown in Legends), while Supergirl suggested that Superman's been publically active for a long time.

Aside from which The Flash was already establishing the idea of the Multiverse for storyline reasons, so it was a good way to allow Supergirl a way into the story if it proved successful.