Was anyone ever invested in Supergirl though?
I'm told she's had some well-liked stories.
Still, James Gunn seems to have this obsession with pulling up C-list characters from the comics and putting them in central roles in movies. For a lot of the running length of Superman, it was a movie about some guy named Mr. Terrific that nobody's ever heard of. I assume this is because he wants to tell new stories, rather than rehashing the same old origins and motives for characters that everybody's known about for years. But it's not the same as actually introducing new, appealing ideas; these characters are C-listers for a reason. Nobody cares.