Bigger caliber doesnt automatically mean better penitration. It does mean less ammo, heavier weights more recoil and having to justify the cost to Congress.
Still the Army is in the start of adopting your bigger caliber. The thing about this though it we are not just talking a barrier penitrator but a armor piercer that gets more expensive as it demands more specialized materials. A steel cored round will generally not pierce level III or Level IV body armor. That demands a core of tungsten carbide steel. That costs a pretty penny.