I would think that the step-thru method goes back to stringing longbows back in the early days of the ELB. They had heavy, thick limbs and wouldn't be hurt to much by the method if done incorrectly. With the advent of thinner and narrower limbs on both longbows and recurves, the torquing that is possible with the step-thru method probably became problematic which is why we don't use it today.
Of course, this is all conjecture but it seems to make sense.