Bacause there are really zoftick and skilled girls, such as Rei Miyamoto, Saeko Busujima and Saya Takagi. Each of them has beautiful and eye-catching hairstyles and costumes, plus the imposing weapons, like gun and sword.
Hence, as for cosplay girls, such kind of sexy and cool anime girls are quite atteactive. And we could find that there are a lot of cosplayers are into Highschool of the Dead cosplay, especially these three main girls.
It looks like that any of them cosplay Rei or Saeko and Saya looks quite sexy and noticable.