I can't say I am right but I always figured that the 4bbls of 50's early 60's era were kinda small on the CFM side and to make big power on big cubic inches, you needed two or more carbs. By the time the 440 came out there were also other big cube engines so the Carb companies started making bigger carbs for them. With bigger carbs available the need for dual 4's wasn't there for daily driver's. I doubt the Hemi or 6-PAck engines were ever considered grocery getters or you would have seen more than just the two or three 4 door hemi cars made.

That has always been my take but I am probably wrong.