Why does alexander pope use the style and narrative form of epics to describe a relatively insignificant incident in “the rape of the lock”?

Because "The Rape of the Lock" is supposed to be a mock-heroic narrative poem. Pope wanted to write about something insignificant as stealing a lock of hair in such a grandiose style of the old masters. It's a satire, intended to emphasize the triviality of the incident, a burlesque.