Is it because of the doubles built on this cartridge, or the cartridge itself. I could see the advantage if the rifle was a slim 9 pounder, but would be piggish in full sized double.

If the double was in the 10-11 pound range, then the 450 3.25" is superior in my opinion. You can load it from to 450 Nitro for Black with a 350gr projectile to a 500 grain full Nitro hammer. What flexibility.