Christ fulfilled the Law. Since we are in Christ, the Law was fulfilled for us who are in Christ. We fulfilled the Law because Christ fulfilled the Law.
All the ceremonial and governmental laws of the Old Testament are fulfilled in Christ because they pointed to Christ. Therefore we no longer practice those ceremonies and regulations, because we have Christ.
All the obligations of the moral Law were fulfilled in Christ, therefore the Law has no power and purpose for us as a provider of righteousness.
If the moral Law has no ability to make us righteous, there is certainly no point in adding anything to the moral Law for the sake of righteousness. If the God-given, holy and good moral Law holds no power and purpose to make us righteous, then all extrabiblical (things added to the Bible) and unbiblical (things that go against the Bible) traditions and commandments of men are also useless for righteousness.