The 45\70 and the .45 Colt both are rimmed designs. Head spacing occurs on the rim. This allows for a good solid crimp that allows for a secure hold and proper ignition. With the taper crimped .45 ACP, head spacing off the case mouth might not cut it for day in, day out service.

Daryl is right when he suggests that the bullet variety available for the bigger rounds makes them more useful.