Because they are easy to animate lol
Well i didnt decide this alone, we beleive its still trendy and its a nice playground for the troller and other survalists mixed with the zombies that will mess around.
The main reason is its more easy for us to build this game faster than SWA and we have tons of packs adapted for it. So less works to do for the artists that are hard to find and keep in the team.
Also most of us are gamers and we are frustrated by some functionality of the acuals zombies games we want o make it more fun to play and more inovative without the frustrations we encounter usualy.