I recall this being something to do with the physics involved. Modern ballistic vests don't stop or reflect bullets like a stereotypical Iron Man armour, they're special fabric designed specifically to absorb the energy from fast-moving small things and slow them down to zero. A thrust knife moves a lot slower than a bullet and has a different shape (wider, flatter, etc) so apparently it can go through standard vests without much trouble. It's difficult to create practical vests that can stop both.