Why does the federal government play a major role in protecting americans civil rights

The federal government plays a major role when it comes to dealing with cases involving civil rights violation and discrimination of American. Americans have an option of filing a complaint with the government at the federal level whereby the federal government takes steps to enforce one's civil rights. 
Filing a complaint usually triggers the federal government to investigate one's claims and depending on the specifics of the case, the federal government takes action i.e. ordering a mandatory mediation of a dispute.