Why did Emily Brontë choose two narrators for the novel?

Wuthering Heights has more than two narrators, if we include, for example, Catherine's diary entries and Isabella's letters, but the two main narrators are Lockwood and Nelly Dean.
Bronte chose Mr. Lockwood so could frame the story from the point-of-view of a complete outsider to Yorkshire, the Earnshaws, and the Lintons. Lockwood is a wealthy, conventional, and somewhat clueless man who is probably meant to be like most of us, in that he is not driven by the fierce passions that run through the main players in this family drama. He, like the rest of us, is also not used to the isolation of the moors and the inwardness and intensity it causes in relationships. We see him in the beginning of the novel gradually trying to make sense of the hostility that seethes all around him. Like him, we go from being initially repulsed to gradually being drawn deeper and deeper into the story of a long ago love affair between two neglected children.
It would have been implausible to tell the whole story through letters and diaries and would have lead perhaps to too many confusing points of view. However, Lockwood, who has gotten sick, has to find some way to finish the story from Catherine's diary that has so excited his interest, especially after he dreams of Catherine trying to break in through the window. Nelly Dean, the servant, is the perfect vehicle because she is both an outsider and an insider. She is an outsider because she is a servant and not a member of either the Earnshaw or Linton families. She is an insider since she has worked for both families and because she has been working for the Earnshaws since she was a young child. Family members often confide in her and she is an eyewitness to much of the action, so she can tell the entire story of what happened from start to finish. She has also been a player in the story, who, at crucial times, failed to help Catherine Earnshaw. She is a fascinating narrator because she knows the story and is a smart, practical person who has lived by her wits but also because she slants how she tells the story to justify her own actions. This means that, rather than gloss over the bad qualities of her employers, as other servants might, she highlights them so that things she has done, such as not tell Edgar how sick Catherine is, don't look so bad. She is unreliable, motivating readers to think for themselves about the events of the story, 
In the end, the two narrators allow us to learn the whole story and from the perspective of two people who are not a direct part of the families in question.