Anybody who even study basic economy, will tell you, there are three type of economic, business economic, public service economy and charity. All three are important. the reason Communism don't work, because, it jumble all of it up and refuse to change. certain thing need to be business, others need to be public service and other need to be charity. Business if for profit, public service is to serve the public, charity rely on public's goodness to help those in need but not cover by public service (usually due to budget restriction)
Private police force will be for profit and that could damage the police's ability to perform police duty, due to its conflict with profit margin. Currently, the police force is a part of public service and must remain so. Not to mention funding a private police force is more expensive, because it is a business, they need to make a profit. A police force under the public service system is not for profit, therefore cheaper to fund.
Maybe in some 3rd world country where the police have no idea what they are doing, the government can pay 1st world nation's police to come assist in policing and help train their police force (if they can afford it), if the 1st world country aim to make a profit, then it would be a private police force, to a certain extent, but things shouldn't be taken further then that