People will pay for a known quantity.

Christopher Nolan puts butts in seats. Spider-man will put butts on seats.

Supergirl is not well known enough to be a must-see, especially when only one rebooted DCU film has come out so far. The track record for the DCU pre-reboot is still likely hurting Gunns universe.

It also doesn’t help that the Chuds crawl out of their basements to complain loudly whenever a film stars a woman.