As the movie goes on, you might feeling like a little kid in the backseat asking, "Are we there yet?" It takes awhile—and we mean several hours—to get to the actual Chinatown. Come on, Polanski: we were expecting a film full of dim sum parlors and jade-filled boutiques.
In fact, against our expectations, barely any part of the movie takes place in Chinatown. Only the final scene is actually set there. But Jake talks with Evelyn about how he used to work in Chinatown as a cop, saying that he did "as little as possible" on the job.
Apparently, this is because he and the other cops never felt like they really understood what was going on in this neighborhood—it was too culturally foreign for them. He didn't want to do anything because he was worried it would turn out to be the wrong thing, and just lead to more injury and exploitation. Jake gives a cryptic hint about what went wrong, telling Evelyn:
GITTES: I thought I was keeping someone from being hurt and actually I ended up making sure she was hurt.
The screenwriter, Robert Towne, said that he learned about Chinatown from a Hungarian vice cop who used to work there. The cop said, "police were better off in Chinatown doing nothing, because you could never tell what went on there." (Source)
In the absence of law, the people who control money and power end up being able to do whatever they want—like Noah Cross.
The police aren't able to comprehend his strategy, leaving him free reign to work his wicked designs on his daughter and on the city. As the writer Ewa Mazierska put it, "Chinatown became a metaphor for the ambiguity of modern life and 'the futility of good intentions.'" (Source: Ewa Mazierska. Roman Polanski: The Cinema of a Cultural Traveller. I.B. Tauris: 2007. 79.)
The movie's final line definitely supports this interpretation. After Evelyn's been shot and Noah's captured his daughter/granddaughter, Walsh tells Jake:
WALSH: Forget about it, Jake—it's Chinatown.
The word "Chinatown" here means a situation that can't be helped…because the person trying help is out of their depth. Even though Jake's realized the truth about what's happening, he's too late—and the cops are too unwilling to give him a fair hearing. At the end of the movie, Jake's suffered the same nauseating defeat that he initially suffered when he worked as a cop in Chinatown the first time.