Or because the costumed females are not *attractive* enough (according to them). Apparently fandom belongs to men (obv), and these guys reckon there should be a panel of judges to assess wouldbe girl geeks before allowing entry. I imagine points would be allocated for hotness+availability and geek cred; insufficient GC scores could be made up for by extreme hotness, but only as long as availability was also present. Insufficient hotness would probably be an immediate fail, because what's the point of having non-hot women present?

Naturally this applies only to women. No one would bother to question male fan geek cred, and certainly no one would care whether they were hot enough for their costume choices.