Rooney Mara vs. Portia Doubleday

It is kind of interesting watching this movie and seeing how Rooney Mara in a 10-minute cameo role just blows Portia Doubleday off the screen. At first I attributed it to the hot underwear/sex scene (you know, a dorky guy COULD possibly score with a really hot, sex-hungry girl he just met, but he really has better odds of winning the lottery--and there sure are A LOT of "lottery winners" in movies like this).

But after Mara was in "The Girl with the Dragon Tattoo" and "Ain't Them Bodies Saints" and Doubleday was in "Carrie", I realized something. There both very pretty. They both have a believable "girl-next-door" quality to them despite being very pretty. They both have not-very-talented older sisters who are also actresses. But Rooney Mara is actually quite a GOOD actress while Portia Doubleday could be replaced by a cardboard stand-up of herself and no one would notice. Why was she the lead in this?