In "Harrison Bergeron," why does it seem that the characters and story are deliberately one-demensional?
Because of its satiric nature, the story is underwritten without much exposition or deep delving into thoughts and motivations. The characters are stereotypes: for example, Hazel is perfectly average, and so seems dumb compared to the reader; Harrison is an uberman, perfect in every way, but unable to resist the relentless evil of the government.
Harrison thrust his thumbs under the bar of the padlock that secured his head harness. The bar snapped like celery. Harrison smashed his headphones and spectacles against the wall.
He flung away his rubber-ball nose, revealed a man that would have awed Thor, the god of thunder.
(Vonnegut, "Harrison Bergeron," tnellen.com)
There are two main reasons for this seeming-underdevelopment of character and story. First, the story might be a satire on the encroachment of egalitarianism into society; by lowering everyone rather than raising the low, society as a whole suffers, and individualist freedom becomes subsumed by total devotion to the government. However, this interpretation is subject to its own criticism: the second possibility is that Vonnegut was deliberately satirizing the sort of story that aggrandized spirited individualism as thematically shallow. If so, the story serves as a satire of one-dimensional "uberman" stories that promote individualism and denigrate societal opinion.