My understanding is that one of the big problems with the French design was that their nuclear reactors used less enriched uranium and need to be be refuelled several times before the EOL of the submarine. This is not a problem for the French since they have their own nuclear industry and easy access the necessary enriched uranium. However getting access to this fuel would have been a big problem for Australia. The US design however is designed to never need refuelling during the lifetime of the submarine and as such removes a significant hurdle for maintenance.