Well, a while back, Ford said it was discontinuing the Crown Vic Police Interceptor too, and police forces started looking elsewhere. Since then they reneged, but I'm sure that those police forces figured that they'd put enough effort into the Chargers that it didn't make a lot of sense to switch back.

Also, Chevy discontinued their police model Caprice back in '96, and between then and 2006, when Dodge introduced the police version of the Charger, the CVPI was the only car in the US specifically made for police work. (Well, Chevy did notionally make an Impala, but it was much lower powered and front-wheel drive.)

My point being that the CVPI was basically the only police car there was for ten years. Before then, though, you saw about half-and-half Fords vs. Chevys. Now that there's another viable police car on the market, I imagine you'll start to see a lot more of the Chargers. Which sucks since it won't be as easy to pick them out in your rear-view mirror.