Actually, he did. He improved the defense, 62-7 notwithstanding.
But rumors was that Jimmy Johnson wanted to trade Dan Marino to get a boatload of draft picks like he did with Dallas. Jimmy wanted to establish HIS team just like he did in Dallas.
However, Dan Marino was Miami and Miami was Dan Marino. Marino not only didn't want to go anywhere else, he wanted the offense to go through him not through the running game. And ownership did also.
Jimmy finally found out that Miami wasn't like Dallas and Miami ownership wasn't like Jerry Jones, who gave him a virtual blank check.
And after three years, when Jimmy realized he couldn't implement his strategy to its fullest, and considering his short-attention span for coaching, he left Miami to do what he always wanted - to fish on his boat in the Florida Keys and talk football when he wanted and cash in on his legacy.