A mixture of cultural norms and legal norms. As to cultural norms, I'll give an example. Go to a Starbucks and note how people don't crowd around the bar area where the drinks come out. People will stand in a wide circle around the bar, because nobody wants people to think that they're not waiting their turn. This attitude is indoctrinated from elementary school, where teachers chastise kids for cutting in line or not "playing fair." That same attitude inhibits corruption. Taking a bribe isn't "playing fair." It's shameful and socially ostracized in a way that it isn't in India or Bangladesh (my family is Bengali). In India, everyone will crowd around the bar because appearing like you're "looking out for yourself first" is much more socially acceptable.

As for legal norms: bribery and kickbacks are prosecuted strongly in the U.S. The federal government being separate from the state governments means that federal prosecutors have no inhibitions about going after state and local government officials. Moreover, low-level things like bribery are easy to prosecute because it's easy to prove that money changed hands.