I am merely a military/tactical gear hobbyist so anyone with actual subject matter expertise feel free to chime in.

In this specific case humvees were particularly ineffective against the IED-based warfare being conducted in Iraq and Afghanistan. Light, fast, vehicles are not particularly resilient to explosives.

Also, while the 20” battle rifle does provide superior ballistics for the 5.56 round, it’s unwieldy and there was a fair bit of CQB during the GWOT. 14.5” carbines were a sort of middle ground that could perform in both long range and short range engagements. Night time direct action raids by special forces even opt for shorter 10.5/11.5 barrels.

I recommend Jeff Gurwitch on YouTube he goes in depth into the history and rational behind equipment evolutions during the GWOT from a first hand perspective as an ex-SF soldier.

MRAPs: you need better defense against IEDs.

M4: a short barrel is easier to use in close quarters fighting, and this outweighs the loss of accuracy at longer distances.

The primary change to HMMWVs is the shape of the hull: they’re now designed in a V shape to deflect the force from explosives away to the sides of the vehicle.

that bombs go boom, and militaries don't have the monopolies on them they thought they did.