Why did the profession of detective in this country basically evolve in the private sector?

Not exactly sure what you are asking -however- detectives involved in the law enforcement area investigate ONLY criminal offenses. Civil cases and/or violations of civil law (if they need investigation at all) have to be done by someone. Thus the profession of private investigators evolved. There are private investigators that work in the criminal area, but they are mainly either self-emplopyed or are on the staff of criminal defense firms who employ them to bolster the defendants side of the case.