Marino always had the arm and the smarts but he never had a Terrell Davis as Elway did when he finally won his two championships. Marino was so good I don’t think Shula and the Dolphins management thought they needed anything else. He came close - losing to a monster 49ers team in the Super Bowl (it wasn’t Marino’s fault that Montana picked apart the Dolphins D that day) and the Pats team in the AFC championship (the Pats team that then got flattened by the Bears).
I don't know why that team has always been like that. If you go back through their history you'll be amazed at just how few good running backs they've ever had.
Do you remember the regular season game against the Jets when the Dolphins got the ball down in goal to go range. Marino and the rest of the offense run up like they were going to spike the ball to stop the clock and the Jets D was just walking around. Marino fakes the spike and then throws a TD pass. Right at the end of the game!