The main reason would be cost due to the 2 piece design of the 2wd manifold.
So will all redesign design it better.

Same with the 200 block it was designed based on the 500 block and due to increased strength to accommodate the 4x4 system. There is a lot of weight bolted to the bottom.

Ford would not really done any modifications for power only reliability in service and cost.

tosh if you was aiming for over 400 and beyond I would say go for the Hart style inlet.

Paul