In the early 2000's, just bad luck. They had great records, just didn't have the luck in the short series. Run a binomial distribution, and you will see that there is a huge chance of the best team not coming out on top in a short series. Make it two or three short series in a row, and the odds are stacked against any team, no matter how good. That is the nature of baseball, where the greatest teams of all-time lose 1/3 of their games. Different than other major sports. Since then, other teams have caught up, and have more money to invest wisely. You think the Yankees and the Red Sox weren't paying attention? Boston hired Bill James himself. And in recent years, Billy Beane has gone a little wacko, going against his own credos in things like the Addison Russell trade and the Josh Donaldson trade (and just now the Rajai Davis signing). BTW, if we knew then what we know now, we would realize that the incredible individual season of 1961 wasn't Roger Maris nearly as much as Mickey Mantle, Norm Cash, and Jim Gentile.