It has to do with the rate of predation a species experiences. A species like a rat has many predators, and thus has to constantly mutate to pass on changes which may help it out compete predators who are looking to devour them. Low predation species have no such survival benefit from a rapid rate of mutation, so they become longer lived. This includes humans, whales and so on.

Essays should not merely portray detailed findings, while missing the bigger picture. Thanks for asking for the so-what.