Because you are messing them by growing at 24/0..............18/6 is optimal!
ALL cannabis plants, autos included, NEED "sleep"!
Without "sleep", a cannabis plant will have a messed up metabolism and a plant with a messed up metabolism will never reach its full potential or have peak health. There are certain metabolic processes that a plant can only do during lights out, without lights out.........you are messing them up.
It is a complete MYTH that it is ok to grow any cannabis plants at any stage of their life at 24/0!