Because the administration of police/defensive-forces is monopolized by the state, citizens don't have the option to hire alternative service providers that might meet their needs better[1]. Similar to a municipality selling a monopoly of ISP rights to a ISP megacorp (read Comcast, Time-Warner), the resulting lack of competitive forces degrades the service. Like most municipal-granted monopolies, the only way a citizen can get choice in those services (public schools, police, road administration) is to be wealthy enough to move, if not you're a serf.

[1] People often confuse: 1. State-monopolized administration of 'public' services. 2. Mandated spending (supplied through tax) with private administration. (This is the model Sweden uses[2])

"People Are Hiring Private Police Squads in Detroit" - https://www.youtube.com/watch?v=4pt6hnabnkU [2] https://www.youtube.com/watch?v=jq3vVbdgMuQ