Those engines have a huge amount of iron in the bottom end for the amount of power they put out. My guess is they didn't need as much oil pressure with so much bearing surface area spreading out the force.
Engineering desicion. You need enough pressure to push enough volume to the furthest point in the system. Any pressure beyond that and you're wasting energy and heating oil from bypassing it through the pressure regulator. The pressure gets the oil to the bearings but after that you need a combination of proper bearing clearance and oil weight and volume for the hydro-dynamic wedging action to occur. High oil pressure won't save a bottom end that's worn out or built too loose.