Crimes are committed against the state. They're defined by statute. They're things like murder, burglary, assault.... The police have to have probable cause to believe a crime has been committed to act, and to do something like an arrest. Criminal cases ultimately are launched by prosecutors, who work for the state. And they tend to file criminal charges only once they believe they have enough evidence to convict somebody beyond a reasonable doubt, and send them to jail if necessary.
Civil cases are between people. They're things like landlord-tenant disputes, lawsuits for financial damages, divorces, etc. It's not for the police to decide on the spot who's right and who's wrong in such situations. They're not attorneys or judges. People need to take such cases to court themselves and go through the court process.
The laws define what's civil and criminal. The police simply are law enforcement. They don't make the laws or interpret the laws.