We always call it the story of "Cain & Abel," but this ignores a critical part of the story.
Eve assumes God will use Cain – she says as much when she first speaks. And we, seeing Abel's sacrifice, assume God will use the younger son to fulfill the blessing of "be fruitful and multiply." But our assumptions are dashed by murder. Cain leaves the scene, and Abel dies. What will God do to solve this problem?
Perhaps the right name of the story should be "Not Cain, not Abel, but through Seth."
Seth's name means "appointed." As in, selected by God to be used for a specific purpose. God's purpose.
This is the nature of things.