I would say that the new truck costs much less to manufacture than the old truck and that is the answer. I believe that this was the reason for the change in the first place. All of that "easy to adapt to Kadees" stuff was just a smokescreen.
That's what you call a penny wise, pound foolish approach. You save a few dollars per car on the trucks and couplers, but lose far more in lost sales. This isn't rocket science.