Why they are not more common in Australia I do not know, I would think at least the English ones would be easy to find.
Off the top of my head there could be a couple of reasons -
Camillus, think of all those clapped out old single shot Anschutz .22s and Browning pump action .22 'Gallery guns' you see at gun shows here. The bores of many of them are nearly worn smooth from a lifetime of shooting bunnies. I can't imagine how many rimfire rounds that would take, but all of us here who shoot, will either remember ourselves or know others for whom shooting and skinning a hundred or so rabbits a night, was in no way remarkable. And they were at it night after night too. There's not a lot of other countries where that intense level of shooting, skinning and field dressing was as commonplace as during the rabbit plagues here.
Those old Rodgers Barlows and short bolster bunny knives would have copped a similar caning through their working lives, been worn out or sharpened down to a toothpick and thrown in a toolbox in the back of the shed to rust or become a kids toy.

Skins being stretched.
Consider that scenes like this were commonplace over much of settled Australia from the late 19th, well into the mid 20th century. And Sheffield, and to a lesser extent, Solingen bladeware would have been put to work on nearly every one of those bunnies.
Also, I was reflecting the other day on this and I recalled a series of visits I made for a previous job to the main state warehouse of one of the major charities in Australia. This organisation is a household name and has op shops ('Opportunity shops' - thrift stores) in every town pretty much.
So every donation from the public is actually picked up from the bins and brought there to be sorted on big tables and assessed and either thrown away or put into the stillage for the relevant category - clothes, shoes, appliances, books, electronics, crockery, flatware, paintings etc.
You name it and it was in this big warehouse.
Now I remember that one of the issues the sorters had was sharp objects - they'd get broken crockery, bits of wire and metal, and of course all sorts of household knives mixed in with everything else from well meaning donors.
What they did with any that weren't serviceable blunt table and butter knives was throw them all straight in big Sharps bins next to the tables. I'm not sure if they sold the metal for scrap, but I'm willing to bet it all went to the rubbish tip.
I didn't see any pocket knives go that way, but I bet that many, many of them have.
Think of all the well meaning older crowd, particularly in country areas, who in gathering up a bunch of stuff for charity would look at some worn, but still usable old pocket knife lying around and think 'some young bloke might get some use out of that', and put it in with everything else.
Well, like it or not, OHS standards mean none of them will ever get to the shops.
And that's why I never bother to look in Op shops for old pocket knives. I have seen some nice Sheffield flatware in them, but that's another story.
So, as I said, I've wondered exactly the same thing, Camillus and Augie, and I figure that might be a couple of reasons why. There's obviously something going on, as Sheffield and Solingen were pumping Barlows and short bolstered Bunny knives into Australia as fast as they could make them for a long time.