Why? Because John Barrowman now has a deal that makes him a series regular across all of the CW's DC shows including Arrow, The Flash and Legends of Tomorrow. It's a deal similar to the "groundbreaking contract" Wentworth Miller has that allowed him to star as Captain Cold on The Flash and Legends of Tomorrow. TV Line, which reported the news, doesn't specifically say the new-to-the-CW series Supergirl is included, but it does specify the deal is for all "all of the superhero shows executive-produced by Greg Berlanti" of which Supergirl is absolutely one.
Still, even if he just returns to mess with the Flash or joins the new crew of Legends of Tomorrow, that's a lot of scenery to chew, even for John Barrowman.