Here's Why Grant Gustin's 'The Flash' Was Chosen For 'Supergirl' Crossover Before 'Arrow'

There is much excitement over the upcoming Supergirl/The Flash crossover, where Grant Gustin's Barry Allen is set to help out Melissa Benoist's Kara Zor-El in defeating the Silver Banshee (Italia Ricci). But why did it take so long for the crossover to take place? And why didn't the producers decide to bring in Stephen Amell's Oliver Queen from Arrow?

According to Deadline, the cast and crew of Supergirl recently took the stage at PaleyFest, where they answered several questions about the show. When asked why they chose to have Barry in the crossover event, executive producer Greg Berlanti had an excellent explanation.

"We needed to create a way to do it," Berlanti said, adding that it was necessary to make sure that all the heroes could exist in the same universe.

"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," he said, admitting that they opted to go with Barry because the Speedster's show finally opened up to other worlds.

"We introduced the multiverse in The Flash this year, and that was always our way in," Berlanti concluded.

There are also speculations that Arrow wasn't chosen for the crossover since Oliver is still set on fixing Star City and probably won't be too eager to help a flying alien just yet. However, the fourth season of the show has already started to introduce a more fantastical element that might hopefully give way to a Supergirl/Arrow crossover.

Amell had previously spoken up about the possibility of a crossover with the CBS show.

"I would love to go over to Supergirl. That would be a blast, if for no other reason than I miss shooting in Los Angeles," he said with a laugh.

The Supergirl/The Flash crossover event will air on CBS on March 28.