I'm not sure when it started (one source gives 1938) but I'm fairly certain that the 34" outside width of frame rails is a SAE standard, established to permit interchange of bodies between chassis of different manufacturers (to save body makers from having to make different bodies for Ford, Chevrolet, Dodge, etc.). In the same vein, the distances from back of cab to rear axle are also very close to standardized - for the same reason.
One reason that the lighter modern pickup trucks still have wider frames is that they are pretty much designed to have proprietary bodywork and single rear wheels rather than specialist manufacturer bodies and dual rear wheels.
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