my guess is IGF-1 is probably one of the biggest reasons, male castration will lower IGF-1 permanently, there's already been other studies showing both animals and humans with lower IGF-1 levels tend to live longer than those with higher levels

IGF-1 can promote cancer growth. (I don't think it causes cancer, but improves the environment for it to grow)

Testosterone plays a role in stimulating release of IGF-1.