Electric locomotives were usually (lots of exceptions, no doubt) intended to work alone: one unit per train. Double cab (or center cab with equal convenience for operation in either direction) thus operations: no need for turntable or wye between runs. American freight diesels-- starting with the FT-- are intended to be used in multiple, so two cabs/unit is unnecessary. (Back in the early days of dieselization, some units were built with 0 cabs: B-units.) And cabs are expensive.
Note that railway systems with lots of single-unit operation-- think Britain, Europe-- have many two-cab diesels.