some topics die because they have been discussed to the point where there is nothing more to talk about, which is more like the topic "ending" rather than "dying". Other times, people just don't wish to discuss something or mean to wait until later and just forget. In some cases, like with the rpg I am in, someone will get busy and will not be able to reply. By the time they ae able to post, everyone has moved to a different rpg and no longer wishes to continue the old one.