Chicago exerts a powerful influence on each of the characters in Sister Carrie primarily because it represents an economic engine that either takes its inhabitants up the economic scale or down. The novel is rooted in urban life, urban temptations, urban successes and urban failures.
When Carrie arrives in Chicago, she is immediately drawn to the possibility of upward mobility that surrounds her. For example, as soon as she realizes that her sister and brother-in-law are relatively low on the food chain and take no measures to move up, she becomes critical of them--not because of their natures but because they are not trying to better themselves. After her disastrous stint in the shoe factory, being treated with unaccustomed familiarity, she understands that her only course of action in the city is to move into the upper-middle class or, even better, the upper-class stratum of society.
Because she is surrounded by all the goods and services, produced by the city and available to those who can afford them, Carrie understands that she needs to do whatever is necessary in order to become solidly upper-middle class. Her decision essentially to sell herself to Drouet, for example, is her first significant step up to the American Dream as a construct of the urban environment. She understands intuitively that her path into society is through Drouet's "support." On a moral level, Carrie is completely aware of the implications of her decision, but in this very deterministic world, once she has made the decision to move into higher levels of society, her path is set out for her, and morality becomes a secondary (or even tertiary) consideration.
Carrie's desire to move out of blue collar society is based not on an innate desire to be in a certain class but is the inevitable result of the myriad temptations provided by life in such an urban setting as Chicago where class is determined by the goods and services to which someone has access. And Carrie has decided that she must have that access.