Okay Paintball Guy, you have asked a fair question, but I don't think there is a fair answer.
For one thing, this is Canada. The .45-70 (which fits this action beautifully) was never regarded as much of a "Canadian" cartridge. When I started shooting (just about 50 years ago), .45-70 ammo made in Canada already was collectors' stuff. Any .45-70 ammo around was imported from the USA, either Remington-UMC or Winchester-Western. Canadian was rare already.
On the other hand, every hardware store in the country had a box of.43 Mauser. It was $3.65 a box as compared to $5 or even more for the .45-70 (if you could find it) and it was every bit as good a cartridge. The .45-70 was a .45-cal cartridge loaded with 70 grains of Black and a bullet anything from 350 through to 405 to 500. The .43 Mauser was a true .44 (bullet diameter is .446) with 77 grains of Black and a 386-grain bullet at significanly higher velocity than the .45-70.
The.43 was here, it was common, it was available, it was smokeless and it was a BETTER CARTRIDGE than the .45-70. As to .45-90, which would have been interesting, it already was collectors' status only.
The .43 Mauser dropped in popularity for many years, finally almost dropping out of sight when all those War Two rifles hit the market. By about 1965, Dominion was the last company in the world making it.
And that was just about when modern milsurp collecting took off. The Americans realised that there was Something Good to their North: a factory which was making all the old-time calibres which had gone out of style in the States: .25 Remington, .32 Remington, .219 Zipper, 6.5 Mannlicher-Schoenauer, 6.5x55, .22 Savage High Power, 455 Colt, .22 Winchester Rim Fire, .22 Remington Special, .22 Winchester Rimfire Automatic, .22 Remington Auto, .32 RF Short, .32 RF Long, .25RF, .25 Stevens RF, .38RF Short, .38RF Long, 32-40 and a PILE of others. And .43 Mauser with a slight alteration to enable it to fit all those Spanish Remingtons that were on the market (and that we couldn't get).
It looked as if the .43 had an assured future.
Then IVI bought out the old Dominion Cartridge Company and all they gave half a dam for was the police and the military. They scrapped all existing stocks of old-time ammo and didn't even honour orders which were on the books for ammo and components. They listed only TWO pistol calibres, .38 Special and 9mm Luger..... and would not sell 9mm to civilians. And they chopped the rifle cartridges list by better than 80%, including the .43 Mauser and an entire LINE of really good .22 Match ammo. After all, when you've got the Government in your pocket, who cares about the stupid Canadian public? THEN they started making ammunition which was plain dangerous, then spent the following 20-odd years trying to live that one down.
And that brings us to today. The .43 Mauser is still a better cartridge than the .45-70 but it's not available... and it would cost a ton of money to rebarrel a Mauser to the less-useful American cartridge. A .45-90 conversion might fly, or possibly a .40-82 but, in both cases, you are up against the brass problem. Easier just to shoot them as they were built: as a true .44-77-386 which happens to have ".43M" stamped on the case-head. And you can duplicate that old Dominion smokeless load with 29 grains of SR-4759 and the correct bullet.
Hope this helps.
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