We have to consider that most of the starting zombies are actual new players or people that are still trying to understand how the game works. We have to consider that mostly of the humans killed either go AFK until the next map or leave the game.
By what was said above, it means that mostly of the times, playing as a zombie is not fun, specially for those who don't yet understand how everything works or just simple find that the zombies lacks in content on the first waves.
By adding this, I would think we would probably have to add a new zombie class or zombie classes (more than one) or really make the purchase of this armor expensive on the worth menu on the point of people having to either pick up it or the items they always had.
we have to remember that humans have range attack since the first wave, while zombies are mostly melee, they either have to sneak attack the humans or wait until the unlocked classes are avaible to be able to actually do something, and adding this armor, removes comepletely their chance of redeeming or having a successful attack.
If the Z team actually worked as "a team" zombies are indeed good and the human survavibility is lowered by alot, but yet mostly of the time people are just playing it by themselvs.
In resume, giving one more layer of defense to humans would mean having to sacrifice what they could do if purchasing it, or either buff zombies or add more things to the zombie team in order to have a fun gameplay for both sides.
Heres a good exemple, the armor perk on the health kit, the armor it gives just gives the player alot advantage and reaction time to escape death. the Tatical vest as well, it turns the player into something closer to a tank class able to escape any zombie, including bosses.
A good balance for it would be having the player to be slower when having this specific armor equiped for exemple.
The Idea is nice tho.