In response to soaring crime rates and chaos at the municipal and state levels, the Federal government swoops in with the answer:
One does not have to endorse Kirk's wheels-within-wheels theory--in fact, one should not.
But the reality is that human beings abhor chaos and the fear that comes with it. And history shows us that fearful people will do all sorts of things to make the fear go away.
See, e.g., the Patriot Act, Department of Homeland Security and Transportation Safety Administration.
A national police force would sound comparatively mild to people horrified by soaring homicide rates.
But it would be politicized right out of the gate, with historically-predictable results. By the way, the previous link omits the salient act of the politicized Asaltos: their participation in the assassination of parliamentarian Calvo Sotelo. Which, in the self-reinforcing spiral of Spanish politics, made the officers' coup viable.
Likewise, the arrival of a national police force, accountable only to the party in power, would invariably aggravate division, with foreseeable results.
Which, in our times, makes it a virtual cinch to occur.