Madden had the misfortune to coach during an era of two dynasties. In the early part of his career, the Miami Dolphins were the best team in football and he could never quite get over that hump. After the Dolphins, the Terry Bradshaw Pittsburg Steelers were the best team and he struggled to beat them, too.
Those two teams were the reason Madden only won one Super Bowl..
Well there was that certain 'reception', as well.
I too thought he was dead until the hype on the documentary started.
I'm not shocked by anyone the dies over 80 (or 70 for that matter). He had a good life, pretty much did everything he could do, hopefully he was able to enjoy his last years out (he retired in 2009) of the spotlight.