Genres have a history and impose a historical character upon the writer. What is interesting in the poem involves a certain kind of dramatization of the self that you don't have to engage in in the essay. In fact, the essay is a more social medium than the poem.
Vijay SeshadriI could say that in the essay, as it has developed historically, success is determined by the writer's ability to express, through an individual voice, a collective experience - you are speaking individually but you are representing collectively.
Vijay Seshadri