re: why did Jimmy Carter do it?
First of all, don't assume that his specialized training (physics and Naval nuclear systems, as I recall) gave him any particular insight into the waste reprocessing end of things.
Second, I spent some time looking into this question after you originally speculated on it, and found this timeline.
From this source, it seems that Ford was the first one to put the brakes on it in 1976. Carter just drove more nails into the coffin. If you look at the rest of the timeline, the issue seems a lot more complex than I originally thought, and it doesn't seem fair that Carter should be stuck with the entire blame. The whole thing looks like a dog's breakfast.