US reactors use highly enriched uranium (HEU) whereas in their nuclear boats the French employ lower enrichment in their uranium fuel. HEU and advanced reactor design make mid life refueling unnecessary. Early US nuclear submarines often required 2-3 refuelings per hull lifetime.
Separate from this point are the politics of the whole thing, which involves the French, British, Australian and US governments. And the catalysis of the current Chinese governmentís policies and operations of the PLAN in the Western Pacific. Although vastly improved diesels and some impressive AIP systems exist for submarines, I think one consideration was the speed and range of nuclear submarines. That may also have been a consideration when canceling the French submarine agreement.
When Australia decided to get nuclear subs, why didn't they redo the contract and buy the French one?