Because the heroic tragedy arouses only “admiration” and “concernment” an unhappy ending was not considered as appropriate or necessary for it.
There is no place for tragic awe and sense of waste in the heroic play. Dryden discarded the unhappy ending. The aim of the playwright was to extol some great hero and this naturally made an unhappy ending quite unsuitable.
Heroic play is a play offering one sensation after another, arousing hopes and fears, and at last making the event happy to the infinite surprise and wonder of the audience. The hero does not die in the end. He is virtuous, and so virtue must be rewarded. It is only then that the people would follow the virtuous example of the hero. Poetic justice was, therefore, considered necessary in the interest of moral edification.