There are a couple of reasons why Elizabeth has such a narrow scope of judgement. One reason is that, while she was born with an observant nature, she was also born with a critical nature. Another reason is simply that she's young and naive.
We first learn about Elizabeth's observant, critical side after the Meryton ball. During a conversation with Jane about the Bingley's, Elizabeth criticizes Jane's inability to see anyone's character flaws. As Elizabeth phrases it to Jane, "You never see fault in any body. All the world are good and agreeable in your eyes" (Ch. 4). In contrast, Elizabeth very readily sees negative traits and ridiculousness in people's characters. In fact, she believes she is being far wiser than Jane in being able to discern negative traits.
A second reason why Elizabeth has such a narrow perspective is simply youthful naivete. As she soon learns, though she "prided [herself] on [her] discernment," she finds out that her abilities to discern, or to judge, are actually not what she thought they were. Instead, she soon sees that she "gratified [her] vanity, in useless or blameable distrust" (Ch. 36). For example, Elizabeth judged Wickham to be the most amiable man she had ever met, simply because he was a very polite, friendly, conversational man. In contrast, she judged Darcy to be an extremely detestable, prideful, conceited man simply because he is quite, has a strong reserve, acts like he is better than his company, and also, most importantly, because of what Wickham told her about Darcy. In short, had she been older and wiser she might have easily known that friendliness does not equate to goodness and reserve does not equate to meanness.