That’s because you can never know the author’s intent. That’s a very outdated (and very overdone) way of thinking about literature. It’s also one that we have realized isn’t actually very useful for the purposes of most literary analysis. While it can be useful to look at the historical conditions under which the text was produced (New Historicism), there is much more insight into what literature “means” when we look at aspects of the text itself such as symbolism and metaphor, because these aspects can evolve with the text and with time and with the reader, and this unfolds many more possibilities about ourselves and the work of art. There is certainly a place for combining New Historicism with other forms of literary analysis to use knowledge about the historical conditions to inform the interpretations of things like symbolism and allegory, but attempting to ascertain the author’s intentions is a futile endeavor that usually makes for a lackluster analysis.