One reason may be that long-term monogamous relationships can help form environments that will aid the successful rearing of offspring, as the male is present to help raise them, leading to an increased probability of the male's genes surviving to the next generation.
One reason for the prevalence of monogamous relationships may be that they create environments conducive to successful offspring rearing, as the male is present to assist in raising them, leading to an increased likelihood of the male's genes surviving to the next generation.