Because in a turn your outside wheels have to move faster than your inside wheels. so they have to be open (or have an LSD) especially on the road. With a locker that outside wheel has to move the same speed as the inside wheel which makes the outside wheel "skip" along the pavement making for a VERY uncomfortable ride and putting a lot of stress on your drivetrain. Most lockers have a certin amount of slip to them but they will make a lout clicking noise and probably still skip a bit...off road the dirt is a lot more forgiving than the pavement so your wheel will drag more than skip...but you should still really only have your lockers on (if you have the option to turn them on and off as the ones that fit the X have) at low speeds or when going generally straight unless you really need them.