The 6.8mm cartridges were designed to deal with heavy body armor, and it has been proven that the issue faced by the U.S. Military is a common challenge worldwide. Hence, the solution devised by the U.S. Military indeed meets the needs of most combat situations.
The 5.56x45mm bullets have been proven to be severely lacking in penetrating power against body armor. The armor-piercing capability of the 7.62x39mm M43 intermediate cartridge is seriously inadequate. Then there's the most commonly used intermediate caliber 7.62 NATO round; although it is 7.62x51mm and has a larger caliber than the 6.8, its armor-piercing ability still falls short of the latest 6.8x51 bullets.
Why is this so? It's because the 6.8 bullets have higher chamber pressure and faster muzzle velocity, leading to a higher sectional density and kinetic energy.