Because Lord DeGroot demanded proper Scottish attire
Grenade launchers proper were not used until WW2. During WW1, soldiers used their rifles to launch grenades with a cup-like launcher attached at the end of the muzzle, hence why Demoman has rifle in this case and not his usual grenade launcher. Please do not throw jarate at me.
The vest was based on images of bulletproof vests used in the early 1920s.
More to come! Once again, a huge thank you to my audiences on DA and Tumblr. You guys helped kept this series going
For other 1920s class(es):
Demoman/Team Fortress 2 © Valve Software