Oddly enough, unlike Flash, there was almost no reason for Barry or Kara to appear if not just for the fact that the perp Team Arrow went after had an ability or power they couldn't deal with. In fact, The Flash episode briefly had Vigilante, a villain that's been causing mayhem in Star City right now, so it easily could have just been him and there'd be no need for Flash or Supergirl to stop by Star City.
This isn't allow of the new team, though one may be going rogue, but for the bulk of this season, they've been dealing with mostly normal people that don't have powers and have slowly gotten better, though they're still rough around the edges compared to Roy, Thea, and so on.