Cat Grant: All four of you standing there doing nothing, you look like the attractive yet non-threatening, racially diverse cast of a CW show.
Lena Luthor: I'm not a villain. I don't wanna kill Supergirl; I just want her to experience the same hurt she inflicted on me. And soon enough, she will.
Answer: Another part of why the change was that it's cold in Canada in the winter and the old costume didn't protect her from it, whereas the new one is nice and warm.