When it was announced that The Flash would be crossing over with Supergirl, fans were ecstatic. They're still ecstatic. The episode will air before the month comes to an end but one has to wonder a bit, "Why The Flash?"
If you're that one wondering, executive producer Greg Berlanti has your answer.
“We needed to create a way to do it," says Berlanti. "We had to introduce Superman since you’ve heard about him and you’ve heard about Supergirl, but in worlds of Arrow and Flash, they didn’t exist in that world. We introduced the multiverse in The Flash this year, and that was always our way in.”
It was the multiverse which made it all possible. Most fans assumed as much when the crossover was announced with very little details at the time and the suspicion only grew when Supergirl cameo-ed on The Flash as Barry Allen traveled through dimensions.
As for why Barry Allen and not Oliver Queen, it seems even simpler. Oliver wouldn't exactly be a snug fit into National City's mythology while Barry would have a much easier time accepting and girl with super strength and the ability to fly.
The Flash and Supergirl crossover airs March 28 at 8 PM ET on CBS.