The Sig Saur rifle was approved because TRADOC needed a way to force soldiers to actually take just 210 rounds into combat. As opposed to stuffing every possible space on themselves with extra magazines like some kind of crazed ammo packrat.
Until we reintroduce the tactical wheelbarrow at any rate.
It's about China. The U.S. military has not been shy about preparing to best be able to fight in the pacific. As for Chinese body armor, the mass issue might not be great but even if it's just some AR-500 style metal plates that would worry US planners enough to justify at least a partial adoption of spicy meatball rifles.