It is clear that he is very powerful due to his fights with Gol. D Roger. He is also very experienced in fighting and has the confidence to defeat Admiral Akainu (magma).
So why hasn't he been promoted to Admiral? Did he deny the promotion when the option was given to him? Or was it because he lacked a Devil Fruit?
Note: All of the Admirals has a Devil Fruit. Golden Buddha, Magma, Ice, Light.