Well, both words have different meaning. Delay means that whatever event was expected to happend at a certain time will not take place at that time but later, for mainly negative reasons, usually beyond control. Whereas Postpone (from Latin meaning "puttin off for a later time") usually implies that the event that was programmed for a certain time has been put off in behalf of somebody's best interest. Basically there are negative connotations attached to daley, same as there are positive connotation attached to postpone.