Follow the money. Preparing new fuel earns somebody a fee for the processing; storing the “spent” fuel earns someone a storage fee. IIUC, France reprocesses its spent fuel for reuse. If the worthless French can do it, why can’t we?
Because Jimmy Carter is an anti-American asshole, and every President since has been (at best) a wimp.
See Presidents Ford and Carter and their non-proliferation rules that included the prohibition against using spent fuel in breeder reactors.
http://www.123seminarsonly.com/Seminar-Reports/034/22542-Nuclear-Fuel-Reprocessing.pdf