The logistics of distributing one brand or model of equipment over an interstellar civilization that spans 800 worlds is nigh impossible. As a result, certain sections of UNSC space might have less modern equipment. Of course, with most of those colonies destroyed, they need a highly adaptable BDU. The post-war BDU can give that. While they do have ballistic vests, it is still highly configurable as you can see via all the variants the marines in Halo 4 use.