Need the muscle to pierce the armor and flesh I guess. *shrugs* You are trying to punch a needle into someone :)
I live on a ranch. I use needles on my animaks all the time. I could hand a needle to a twelve year old and have THEM inject a critter. For armor it makes some sense, except that needles are generally very fragile. Anyone worth a damn would be going for weak points.
Injecting a needle in a controlled environment and in combat are a bit different. Perhaps the needles used by the injector weapons are bigger and thicker for durability and proper quick injection, thus lacking a smaller surface area where minimum force could be used to pierce flesh. Bigger surface area, more force needed to break through. Also, any armor worth a damn would have those weak points protected lol
*shrugs* All speculation and theory.
The concept should still work if you use the Needler Pistol instead.