Independant suspension was a selling point that you see in ford commercials from the era. To this day, GM puts A arm suspension in its 4x4 1 ton trucks. It might have been a cost saving measure too, but not sure how much cheaper the D50 would have really been compared to the 60.
If you look at where trucks were headed at the late 70s, you can see a consistent drop in power and torque across the board, so maybe they figured it just wasn't needed anymore. That could apply more to the reasoning behind the Dana44 that found its way into some F250s. Although from what I hear, the D50 setup is fairly tough, just not as good as a60. The stance of the truck is also lower for more suspension travel, so from a fuel economy and ride comfort standpoint, it did make some sense.
I've resigned myself to either getting a Dana44 TTB setup with 3.07 gears (would strictly be used for snow). Or throwing in the towel and putting a Dana60 under the front of the truck, essentially throwing away its hypermiling capabilities. Or just leave it as is and live with the fact that it will never do well in the snow (<--most likely outcome
).