Superman existed PUBLICLY on "Earth-S" for nearly 3 decades before Kara revealed herself, and yet Metahumans were a "brand-new thing" when the Particle Accelerator exploded, and there was no organically convenient way to reconcile that inescapable fact.
The only way that SG could've existed on Earth-1 is if the series had been developed without the inclusion of Superman and with Kara being "brand-new" to Earth, neither of which are things that Berlanti wanted to do.
The only way that SG could've existed on Earth-1 is if the series had been developed without the inclusion of Superman and with Kara being "brand-new" to Earth, neither of which are things that Berlanti wanted to do.