Most wounds in WWI wasn't from high velocity bullets, but fragments going under 1000fps
Artillery 73%
Machine gun 12%
Grenade, mortar, bomb: 8%
Rifle bullets 5%
Other 2%
Anything that reduces fragments going into the torso will save a lot of lives. World War I hospital reports also show about 20 percent of wounds were to the head and throat: helmets, when introduced, were said to have saved yens of thousands of lives by themselves, and the French Adrian and British Brodie were not good designs, but better that a cloth cap they had before
IIRC a major source of deaths were due to artillery damage to limbs (and actually simple stuff like first aid splints that immobilised broken and crushed limbs as the casualties were transported also saved a lot of lives).
But you are right would it potentially prevent low velocity fragment wounds to the torso, yeah maybe. But not all wounds are in the torso, not all frag is low velocity and so on (artillery can leave a wide range of wounds*), it's still all a trade off. They issued lots of these vests reinforced with steel plate to Russian troops during the Russo-Japanese war but the balance was still not there in terms of cost effectiveness weight and effect for it to be widely adopted .
Sadly a big factor is just how much time and effort was seen as cost effective for protecting your troops against wounds. However this balance also changes as the rest of the balance changes. e.g once you have something that can stop the bullets they're likely to face as well as other stuff.
Look at the body armour in WW2 it largely similar stuff designed to stop flak fragments in air crew (who you never have enough of, and have specialist training so take time to replace, and tend not to be walking long distances in their kit).
Basically they kept trying to make body armour work all the time even when it wasn't common on the battlefield, but that balance wasn't right.
*teh wounds figures get interesting here, because if it's just number of wounds in total artillery frag can leave a lot of wounds on the same target in the same attack.
Not far off from what current US Army body armor can weigh with all the plates in place.
Right which is lot, however there is another major problem with Brewster over and above the weight, and that is it looks like this:
Anyone fancy moving about in that for more than 5 mins, let alone fighting in that!
OTV gets a bad rep for inhibiting movement by modern body armour standards, but you fancy trying to get a cheek weld in the above, let alone maintaining any kind of situational awareness, or hell going prone?!