probably because they want it to be a defensive cooldown, rather than an offensive cooldown?
"hey look, a pyro is walking up to me from 30m away.. oh look, now he's at 10m.. jee, should I use my KB, or my sprint, or my stun"
Yea i get that part, but when i use it, often times its utility. As a healer, i tend to stun high damage dealers that are hitting the people i'm healing, gives me a 4 sec boost to the amount of health that player is getting back. Another neat trick is stunning the snipers sitting on edges in huttball giving ball carriers a chance to jump to them, with good communication of course.