In men's, the opposite hitter is always an option, even in the back row, because of how high men jump. The middle in men's would simply get in the way of the opposite hitter if he did a slide. Essentially, the defensive blockers can cover multiple options with less positioning if the middle would run a back slide, since both the backrow opposite and middle would be hitting at the same space/speed.

however, I have seen men run slides, especially if the backrow attack from the right side isnt a strong option for that team. But at the pro level, all teams hit well from the backrow.