There are many different reasons why the Crown Victoria is much more popular than the Impala, but the main reason is that the Impala is a FRONT-WHEEL DRIVE car. They're just not as durable. The Impalas are much more cramped and rear-visibility sucks. Now, I did not test the Impala for my deparment all those years ago, so I couldn't tell you the real reason my department in particular doesn't use them. But simply putting it, the Crown Vics are more powerful than the Impalas. As for those money-strapped departments, here's other reasons to take into consideration:

The maintenance costs for the Impala are too high. Part for part the Chevy parts are much more expensive than the Fords. Plus, as we all know, the Crown Victoria has not changed much since 1998 and have a lot of parts interchangability, so when one gets wrecked they can reuse the parts. Also the Chevy does not have a frame under it, if it gets wrecked it's done for, the Fords stand up better to accidents and are easier to fix.