[84LX89GT]

I can't give you a definate answer, but i believe the reason ford does this is to provide a camshaft that will work for many different engines in many different cars and keep emissions compliance in all of them, also some vehicles have better exhaust systems than others (even though the heads have low exhaust flow). The 5.0L engines weren't very heavily engineered until the Cobra/5.0L explorers came around even though earlier engines had some measures to improve fuel mileage, power and economy (such as roller lifters, shorter piston skirts, and low drag rings). If ford was to increase the duration of the exhaust the vehicles wouldn't retain as much exhaust in the cylinder to heat up and vaporize the incoming air/fuel and it would burn a smaller amount at closed and partial throttle, and increase emissions.
This is my guess, i could be wrong, but it's my opinion