Also,,, While the 45 Colt was a govt issue revolver round, it was straight cased and originally had a fairly small rim.
So in the original black powder configuration, the straight sides did not seal the gases as well as the tapered case 44-40 and 38-40s (44 and 38 WCF) AND, the original small rim made positive extraction questionable when the chamber was fouled with black powder residue.
You will note that the modern 45 Colt cases have a groove above the rim which aids in positive extraction. But they could not make that in an affordable case back in the 1870s. So they used the balloon head method.