6v batteries are designed for Golf Carts and are real deep cycle batteries. most of the 12V batteries are combination of starting batteries to Deep Cycle.
I think it had to do with the size of plates, thick plates last longer for deep cycle, but don't produce the instant current needed for starting. thin plates make higher current but don't last as long in deep cycle operations.
the only argument against 6v has been "if you have one battery die while your out, you will end up with no batteries because 1 6v don't help, but if you have 2 12V batteries you can pull one and still have 12v.