The entire point is, the M16 was preferred for combat, and M14 based guns aren't. So says the DOD.
It's more than putting a bullet downrange, but if that's all you need, a 10/22 can do the same. What weapons experts decided was to move away from the old design.
Wood stock? Subject to breakage at the wrist, accuracy requires carefully mounting the action, which a beater military gun cannot do. It must be easily dissassembled.
Open top steel receiver? Scopes won't mount over them as case ejection was given priority. The bolt locks into them, the barrel screws into it, making them a stressed part of the gun. The long, heavy construction is made flexible by mounting the stock directly to it. The operating rod is exposed and guided externally. The bolt is almost completely exposed to the elements.
The forward gas piston pushes on the operating rod, but that means the cylinder pushes on the barrel. If not coxial, it introduces barrel bending at just the point the bullet exits the barrel. Since combat guns are 2MOA, it's acceptable, but for precision shooting, who takes the top ten places in the last ten years? DI. No barrel bending.
The piston being separate from the bolt means cleaning it separately, taking longer to do so.
The sling is mounted to a barrel band, same for both Mini 14 or AR, the latter you can free float and add more rail as needed. Mini? Screw a threaded fastener into the stock, no rails or options, and you still need the band. Eliminating it seriously detracts from hard use.
The exposed operating rod and fixed charging handle reciprocates, which interferes with shooting next to cover or concealment. It's not ambidextrous, either.
Either will shoot a paper target or a bunny. The Mini 14 will not be as effective in combat, the entire genre of wood stocked open top exposed op rod guns was passed over decades ago for military use. No one issues them today for general use by all their soldiers, airmen, sailors, or Marines. It's a specialty weapon in the US, largely because we still had some, refitted them, and sent them to SWAsia, where they aren't being used much. It's a ratio of over 100,000 M16/M4's, vs. 5,000 refitted M14's with all sorts of AR enhancements.
The Mini? Ranch Rifle is exactly where it's best used, mount a rack in the tractor or farm truck, it's good enough to replace a lever action.