Why is Gen 3:15 called the protevangelium?

It is thought to contain the first mention of the gospel, of the promise of the final victory of Jesus over sin and Satan. Assuming that “bruise thy head” and “bruise his heel” are not just talking about poor snake-human relations (contra Josephus), the language is figurative; so what does it stand for? Since this serpent is responsible for bringing sin and disaster into the world, it represents the source of sin; and we must look for a descendant of Eve who is thought to defeat the very source of sin. That, of course, would be Jesus (traced precisely at Luke 3:38). And this is what at least one later Biblical writer, Paul, believed: he says in the last chapter of Romans, “And the God of peace will crush Satan under your feet shortly” (Rom. 16:20), and to the Corinthians, “O death, where is thy sting? … The sting of death is sin… But thanks be to God, which giveth us the victory through our Lord Jesus Christ.” (1 Cor. 15:55-57)