A couple reasons. First of all, most police departments prefer body on frame, rear wheel drive cars. This pretty much leaves one choice, the Ford Crown Victoria which wins by default. Also, since the entire police vehicle industry is built around this single model, it is hard to pick another car and get all the lights, bull bars, etc that police cars need to have. Cost is another big reason, the Crown Vic is cheap.
Different cities have different standards, though. Another reason is politics. In kentucky there are Toyota Camry police cars, because they are made there. Vail used to have Saab police cars, though they recently switched to fords.
So not all cities in the US use american brand cars, many use cars that meet the DOT's standards for domestic content (>85%) so they are classified as american cars. Interestingly, the Crown Victoria is technically a foreign car since it has lower than 85% domestic content. So a Honda Accord is classified as American where the Crown Victoria is not.