But they've already said that EVERY OTHER EARTH has a Supergirl. So now you have to explain why all of the rest have them and this one doesn't. That's not so easy to explain away. Now of course, there are all kinds of legal reasons why they do the things they do, but those are external to the story and the story has to make sense. It just doesn't. People watching the show don't care about the behind the scenes minutia. That's the difference between good writing and bad writing. Good writing answers the questions. Bad writing makes excuses for them.
They said every other Earth has a Supergirl, yes. However, the when of it is what's up for question.