Board Thread:News and Announcements/@comment-807326-20161221093022/@comment-2054358-20161221181355

CBS didn't want Supergirl to be in the same universe as Arrow and Flash. It sure took a lot of convincing from Greg Berlanti to do a crossover, which is why Supergirl is on Earth Thirty-Eight instead of Earth One. And also why there was a separate wiki for Supergirl.