They rejected Billick because he is not a very good coach. If not for Ozzie Newsome's defense, he would have been far below .500 with the Ravens. He was brought in from the Vikings as an offensive guru, but failed to get Baltimore's offense above 14th in NINE years. He was even given control over drafting offensive personnel, and drafted Kyle Boller even though Ozzie Newsome did not want him. Furthermore, he completely lost control of his locker room before he was fired, and players were openly criticizing his ineptness.
He's better than Jauron, but that's not saying much.