I think two reasons. If reactors can't function above 20%, a country having access to >20% enriched payload is a certain violation. Vs "60% enrichment is still for clean energy, my reactor works with it". 2. If you are only buying the payload and not enriching it yourself, you can't do anything with >20% . More like mixing methyl alcohol in lab available ethyl alcohol, to deter lab techs from mixing water and having a rager.