It's a source engine FPS that's even more customizable than the others, and they are all pretty damn customizable. Beyond sprays and mods TF2 has cosmetics, name/desc tags and just a bunch of tools to customize stuff in general. If people for some reason truly wants their favorite class to have something in common with their favorite technicolour horse/their furry character, they have a lot of tools to do so I guess?
The bigger question is why does so many bronies seem to play scout, and so many furries seem to play pyro.