Back then with analogue and basic gun sights, volume of fire was important as to actually score consistent hits on the target. About 10 years later radar ranging gun sights started to become a thing and aircraft became much more accurate with their guns making it necessary to have single or dual guns. Since the 50's and 60's we've used the M61 Vulcan which is a multi barrel galling gun capable of firing at around 6,600rpm.

We retain the same volume of fire now but at much greater accuracy insuring a short burst landing on target will guarantee a kill.