Why is Team Fortress 2 so appealing? I think it lies in the character designs. The guys are manly men. There ain't no bony emo-girly-boys with long/spiky hair with existentialist problems, as far as I can tell. Even Scout is manlier than the stereotypical teenage/young-adult protagonists in many other games.
Pyro was omitted because there's not solid proof whether Pyro is female, male, or something else. Once VALVE releases official information, Pyro will be left out of this stamp until further notice.
Team Fortress 2, TF2, and all related characters © VALVE