Why don't soldiers wear a totally bullet proof suit which would cover from head to feet? by Andy Duffell
Answer by Andy Duffell:
Because survival is about more than just being able to resist hits.
This is the Onion Model:
What your armour tries to do (in the diagram: Don't be…penetrated) is actually pretty low down the list of preferred options. Crucially, past a certain point stacking more body armour onto the PBIs makes them less mobile and less stealthy, which makes them more likely to be detected, seen and hit.
So there is a point at which adding more armour actually becomes counter-productive. The guys are probably wearing about as much as they can realistically be expected to carry as it is.