Same exact reason most people do not rank Drew Brees in the top 10 best QBs of all time, even though he's on the verge of having similar numbers to Peyton Manning (who is firmly in everyone's top 5).
Super Bowls get you noticed and increase your legacy exponentially. I don't necessarily agree with it, but that is the stark reality.
What does Drew Brees have to do with defensive coordinators utterly failing late in 4th quarters in playoff games? Absolutely nothing, but it tarnishes his legacy greatly. Again, very unfairly.
The minute Drew Brees is raising that Lombardi over his head again, everything changes. Most will start to say he should be ahead of Manning because he played far better than Manning in the playoffs. They'll start to say he's better than Marino, Favre and Elway. Some will vault him all the way to 3rd best of all time. These same people that have him in the teens right now.