User blog comment:ShroomstagramUser/Question/@comment-25206252-20150302053643/@comment-24971793-20150306165743

I never said popularity is the dominant factor but I do believe it plays an important part, especially for the main protagonists. In the first 3 seasons they still took (small) risks with their final two, but from Revenge onwards they always went for populistic characters.

Dawn's popularity with the fans can also be seen as a reason not to pick her. They choose Zoey as the main protagonist so the creators want the audience to support her and a massively popular character like Dawn (who would have been filler) would have probably overshadowed her. Of course popularity isn't the only reason and the ability to use them in storylines is also very important, amongst several other reasons.