> Why isn't there a Judicial Branch agency that safeguards the Constitution from the other two branches??
Because there is no authority for such an agency in Article III, and because the type of ombudsman agency you describe would be undertaking investigations of a sort that are executive, rather than judicial, so that even if Congress attempted to create such an agency, it would be (were it Constitutional at all) an Article II agency subject to the President even if Congress gave the Judiciary the power to appoint its officers.
Because there is no authority for such an agency in Article III, and because the type of ombudsman agency you describe would be undertaking investigations of a sort that are executive, rather than judicial, so that even if Congress attempted to create such an agency, it would be (were it Constitutional at all) an Article II agency subject to the President even if Congress gave the Judiciary the power to appoint its officers.