Well, for starters, we have to look at the symbolism of Chinatown. What does the area itself represent? Within the story, Chinatown is the neighborhood in which Jake, years earlier, failed to protect a woman. Not only did he fail to "save" this woman, he ensured that she would be hurt by his very attempt to save her.
We can divine that Towne's screenplay contains a larger symbolic purpose for Chinatown. In my opinion, Chinatown represents the power structures of the world, and the futility of attempting to subvert them. The line, "Forget it, Jake. It's Chinatown." is an admonishment to those who seek to do what is right in the face of unassailable evil and corruption.
Noah Cross personifies these malevolent forces. You may recall the scene where Noah and Jake are eating lunch at Noah's farm, and Noah tells Jake: "You may think you know what you're dealing with, but, believe me, you don't." This line, which drips with subtext, reveals that Jake is in over his head, and Noah knows it. Noah has the arrogance of a man who has taken everything he's ever wanted, and nothing has stopped him. He is the power of the world, and the forces of good are no match.
Jake has once before tried to fight back, in Chinatown. The memory of Chinatown is a traumatic one to Jake, for it bears the realization that one is powerless in the face of the world's oppressive evil. At the climax of "Chinatown," Jake is once again reminded that he is grossly overmatched. The police, who arrive during the final showdown, simply enable the corruption to fester right before their eyes. And, just as it happened before, the victim of Chinatown is an innocent woman. "Forget it, Jake. It's Chinatown." - the line spoken by one of Jake's P.I. pals, is a plea to Jake to accept what Chinatown is: a place where good only begets bad; a place where things do not change.