The 1950-ish Supergirl was a clone of Superman for girls. Superman, of course, is "Truth, Justice, and the American Way". Apple pie and all that. In the edgy 90's, some fellow super heroes even called him the Big Blue Boy Scout.
Tom King, in Supergirl: Woman of Tomorrow, wrote a story that really differentiated her from Supes. This movie is loosely based on that.
They just didn't want a female version of the same hero.
I'm looking forward to seeing it this weekend, hope it works out. Between Creature Commandos, Peacemaker (and Vigilante!), Suicide Squad, and this, we have the strangest (and best?) collection of superbeings ever. I hope it works.