It's probably because 는 is the topic and more generally speaks to women where as 가 would be more specific to a woman. We would need more context to be sure but here we have no context.
I think @ jaisaige say thanks to duo as duo states women wearing glasses are beautiful. And may be she wears glasses. Her profile is also of a girl wearing glasses
@duolingo: see? here you accept 써요...but not in the other sentences of the same exercice. Can you please explain what is the grammatical difference? thank you!