In fact, when the Australian government signed the contract they deliberately chose to go with a conventional sub, though the option of nuclear was on the table, and the design they chose had not actually yet been built.
Now, what I know about naval defense could be written in sharpie on the back of my fingernail, but the variant of this story that I’ve heard (couldn’t find a specific cite in a brief google) was that the variant we chose was specifically tailored FOR US because they government said they didn’t want nukes. The linked article kind of implies that but doesn’t say it outright.
If that’s true then we were the very definition of a toxic customer, and if I were the French I’d be absolutely steaming. It’s one thing to say “you’re taking too long and costing too much money” but “you’re taking too long and costing too much money trying to fulfil a difficult requirement that … turns out we’ve decided we don’t actually need” - that puts the clusterfuck right back on us.
I have seen no material evidence nuclear was available. It is actually very rare for any country to share its nuclear reactor designs and technology even with close allies. Without some compelling evidence France was actually willing to sell its nuclear reactor technology I do not personally believe they simply could have bought nuclear.
What I believe more accurately happened is Australia could choose between buying proven French diesel electric subs, which are a known quantity and have been sold to other countries, or buying the French nuclear subs retrofitted to instead be diesel electric. The only meaningful advantage of that design is basically political—some defense experts Aussie side had long said due to the large sea area Australia has (third largest in the world, and the 4 largest all have nuclear subs to help patrol theirs, other than Oz) that they needed nuclear, but back in the Malcolm Turnbull era the Australian public was still too anti-nuclear for that to be a viable political option. Essentially what was done was a purchase order for “uncontroversial but very expensive diesel subs”, whose platform in theory could be upgraded to nuclear down the road.
It was actually probably a poor idea from the very beginning, given the typical expected lifespan of a sub, and the realistic timeframe it would likely take for Australia to wrangle up the ability to nuclearize those subs it is highly unlikely this plan ever could have produced viable nuclear submarines at a price any Australian PM could get away with.
That’s certainly a kinder interpretation to the Australian government than the Canberra Times’ take, which has the government looking very inconsistent. Very likely your right, and the narrative where it’s our government that took nuclear off the table is either a) a bit garbled or b) a face-saving maneuver because they suspected the French would say no.
I do think, though, that the issue was probably not public support, which hasn’t shifted much in the past few years, but the specific PMs involved. Turnbull and Morrison are both LNP, but from very different wings of the party and it wouldn’t surprise me to learn that they have very different opinions on the merits of making Australia a nuclear country.