People say that because the best players are unconscious associated to their big glory moments..so,as marino hadn't had one of those, he's kind of forgotten...IMO Montana was not as great as Marino though you'll always remember him leading the Niners to the Superbowls and that will enlight his overall success and abilities..
Marino was great, we know that, and for most of us he is the best of all time, cause he played for the phins and nobody knows better than us how good he was...For an average NFL fan, other teams fans, it will often occur to their minds as Marino, the guy that was amazing, but not amazing enough.
If he had a ring this thought would never cross people's minds and 90% of the NFL fan base would say, Marino was for sure, the best ever to play the game.