The brake lines are attached to the frame. The other end to the baster cylinder. The body sits on rubber mounts. The body flexes around. Simply put there is movment between the 2 points. Quite a bit of movment I have noticed. Hard lines will have to flex as the body and mounts move around. This leads to fatigue cracking over time. One of the popular way around this that you may have seen on other vehicles is multiple coils of hard brake line. The movment is still there but the stress is spread out over several feet of tubing instead of a few inches.
Now the rubber lines. Manufacturing wise it is an expensive way to go. Ther is a material change (hard to rubber back to hard). More junction points, more risk of leaks/faulty parts. It is an expensive fix, costing a couple dollars per vehicle to do. Consider how auto makers have been trying to make there product as cheap as possible for so long. The only reason Ford would spend the money on rubber lines is if they had too much flex for hard lines to handle over time. As crude as these vehicles may be at times, there was actually a lot of engineering that went into them. For that reason, I run flex. They built them that way for a reason.
