Because contrary to popular believe, girls do love a more complex character and we especially love female character that have power. Anna is a stereotypical manic pixie dream girl.
Plus her look is basic and forgetable. Elsa's look is more memorable and more glamorous.
That's what happens when you're the title character and you sing the dynamite song of the movie… and you upgrade from a basic Disney princess to a freaking queen…
I like Anna more but I guess Elsa has the more appealing costume and has powers!
I didn't really like Elsa as the whole don't feel anything/shun my sister thing was useless as all she had to do was not aim her powers at Anna. It was nice Elsa addressed her negligence in Frozen Fever though.