Just as I'd prefer the guy in charge of the military to have military experience and know what it's like, I'd prefer my chief law enforcement officer to have law experience and know what it's like. These just happen to be the same office.
The DOJ says the AG is the chief LEO and they have a lot of lawyers and legal knowledge/experience over there...
The Judiciary Act of 1789 created the Office of the Attorney General which evolved over the years into the head of the Department of Justice and chief law enforcement officer of the Federal Government. The Attorney General represents the United States in legal matters generally and gives advice and opinions to the President and to the heads of the executive departments of the Government when so requested. In matters of exceptional gravity or importance the Attorney General appears in person before the Supreme Court. Since the 1870 Act that established the Department of Justice as an executive department of the government of the United States, the Attorney General has guided the world's largest law office and the central agency for enforcement of federal laws.
http://www.justice.gov/ag/about-oag.html