By placing the oil filter after the oil pump, the oil pump can deliver full pressure to the filter.
If the oil filter was placed before the pump (on the suction side) the maximum pressure that could be developed at the oil filter would be about 14.7 PSI (atmospheric pressure). The flow rates would probably stink and you would probably run the risk of oil starvation.
I knew how the system was set-up, but I never stopped to think about the why. See, a simple question can make us think.
As I remember, there's also a pressure relief valve in the system. An oil pump is what you call a positive displacement pump. That means it forces the oil no matter what. The pressure relief valve act to limit the maximum pressure in the oil system. Without a pressure relief valve, the oil pressure would (under the right circumstances) just skyrocket and could blow out a gasket or make other mischief.
Going from memories of rebuilding an engine back in the '70s, but the lubrication system is pretty basic and shouldn't have changed.