As it has been told to me, the 69 was almost too big for the White Pass and it is substantially lacking in crew comfort, being a deckless cab with the boiler literally poking out the back of the cab. Even after its restoration, it has not run much.
The 70 series were conventional, inside frame locomotives. Room in the cab, not out in the weather, or cramped between the cab wall and the boiler. No counterweights to hit the ice along the rails and derail the engine (the same thing that may have sent the former D&RGW 470's home after the war even though they were more powerful than the 70s or 80s.). Those two seem to be the big things, but I am sure that there were others.