I figger this has to be economics. Single exhaust has been the "standard" since times were better. One tailpipe, one muffler, etc. Costs less to produce and less to maintain. Dual exhaust was almost always an option, unless it was part of the performance package as in the case of the Olds 442 (4-barrel, 4 speed, dual exhaust.)
Long ago I was told that the best thing I could do to improve the performance of a Chevy V8 was to get rid of the "strangulation" single exhaust and add headers and duals. I dunno how much truth there is to it, but that was the conventional wisdom of the day (a long time ago.)
