He are some of the reason why not frank

Frank is a follower not a leader
Frank is not Chief material
Frank is part of the swart clan
Frank will do whatever the mayor tell him to do
Frank and Lampron action plan has been exposed
Frank had crazy joe followed
Frank has the keys to all of the secret files in the dept
Frank will keep the status quo
Frank will make sure whortiz comes back to work as a promise to the mayor same promise that colina had to honor.