My guess would be they want to keep their options open.
Season 1 would focus mostly on Kate anyway, like season 1 of Supergirl was on Kara, Superman didn't start showing-up and stuff until season 2. Difference being Kara and Clark have their own cities. If Bruce shows up and is physically and mentally fine, Kate has to move to Coryana, or globe-trotting, or wherever they can put her.
Plus, there's been quite a few rumors about Red Death for Flash, not saying that would be Earth 1's Bruce of course, but between those rumors and Conroy being a version of Bruce in COIE - who knows what they'll do, they don't yet.