It seemed to me that a big factor was the prioritization of muzzle velocity (flat trajectory) and rate of fire, both of which would help compensate for less-than-excellent marksmanship. That carried over to the 1950s and the present day, with the quad-M-39 setup on the F-100 and F-8 and, of course, the M61.
Project Vulcan itself (I recall from history briefings) resulted from post-WW2 retrospective studies of gun lethality, which concluded that the average pilot was best served by something combining the characteristics of a rifle and a shotgun.