Because I believe that if the child is given enough time to attach to the carers in the setting, ideally a designated keyworker, then eventually they will be willing to let their mother/father go without getting upset. This may take a long time though in which the parent will have to stay with the child, and then withdrawal has to be gradual.
All the people who are involved with ds (portage worker, SN coordinator at playgroup, his keyworker at playgroup) disagree with me on this and say crying is inevitable, so you might as well leave sooner rather than later to get it over with, because if you stay the child will think that you will always stay.