The story is told in the past tense because Hughes is remembering an event that happened in his childhood. More than that, however, the use of past tense suggests that the narrator, who is remembering this event, somehow knows better. That is, when we get to the end of the story, and little Langston is crying in bed because Jesus didn't come, the narrator's distance from the story is such that we are meant to understand the experience at the revival as a pivotal moment in Langston's life. Langston's lie in church changed his whole perspective on religion, a change that has resulted in the way Langston the writer, remembers the experience and writes about it.
A shift to present tense, of course, would change everything. In that case, the story would be told by 12 year old Langston; the story would become about his crisis of faith, or his stubbornness in church; the tone of the story would be more fraught. The emphasis would be on the drama at the temple, instead of the lesson Langston learned.