Decorated look

"Pigtails make a girl look like a pretty doll"

Pigtails greatly add to the 'decorated' look of a girl, since they are the opposite of a 'natural' hairstyle and are certainly difficult to overlook.
In this respect, it is interesting to compare ponytails and pigtails. Although at first these hairstyles seem closely related, effectively they are very different. Because of the fact that they are easy to apply and hide a lot of hair from the viewer, ponytails can give a girl a no-nonsense look. On the contrary, the application of pigtails takes a significant amount of effort and the result is a hairstyle which effectively focuses the attention on the hair, by making sure each and every strand of hair is visible to the (frontal) viewer.
The fact that someone else must (at least partly) apply the pigtails could very well subconsciously appeal to those men who like to see women in a passive role.