Several reasons. To begin, police have very long sales cycles that are tied to their funding schedule. You need to make sure that you engage with them when their budget has been assigned but they haven't allocated to much of it. They also are very stringent in making sure they buy the correct product. For most large purchases, they do what is called a "request for tenure" in which companies bid on a contract with a given police department, so you have to almost always compete directly with alternatives to your product.

Purchasing decision are distributive differently depending on the size of the police precinct, but in general it is the Chief of Police.