Now, though, this article quotes Macron thusly:
"Macron said the French offer "remains on the table".
He said France would not supply nuclear submarines to foreign countries, so the offer related only to conventional vessels."
So, if true, then the answer to your question is simply that France would not sell them nukes, when they decided they wanted nukes. (In addition to all the political and financial issues.)
Extremely political. Many viewpoints. Some legitimate French grievances.
In the end, while this is the most detailed breakdown I have seen, there are multiple references to financial difficulties with the deal.
This is an older article, closer to the event itself, and about as neutral/matter-of-fact as I have seen:
There are also the diplomatic aspects of Australia's entering into the AUKUS agreement (which is naturally a contentious subject.)
When Australia decided to get nuclear subs, why didn't they redo the contract and buy the French one?