John Marcotte is a dad with two daughters and a deep love of Halloween. He's also a big comic book nerd (in the non-pejorative sense) and the founder of an organization called Heroic Girls, which advocates for better representation of girls and women in comic books and related media. So, perhaps he was already extra sensitive to the kinds of costumes available to girls for Halloween. Most were very cutesy with short skirts, or basically princesses training to be future Slut-o-weens (which are very fun for adults, don't knock it till you try it), and not really appropriate. Either that, or he saw all too much of this kind of thing:

Sources: Heroic Girls