In short, the A's roster was actually constructed to succeed over 162 games but the compromises he had to make on defense and power pitching to get extra value are particularly fraught in the playoffs, where you're battling the better offensive teams in the league. Of course, the playoffs are a crapshoot (the Cardinals had no business being in, let alone winning in 2011), but the compromises Beane had a direct effect on their heartbreaking playoff losses in the early 2000s.
These are not inherently better offensive teams. That's the whole point. The 2002 A's were a good offensive team because they were better at evaluating offense than the rest of the league.
The reason the A's didn't win the World Series is the same reason the Cardinals won the World Series in 2011 and 2006, but not 2004. It's the reason the Giants won it in 2010. The playoffs are a crapshoot. It has nothing to do with whether or not sabermetric analysis is valid for regular season games, but not for playoff games.
This is not true at all, though - what compromises exactly did they make in power pitching? At most, the team structure would decrease their chances in the playoffs by a few percent. Any analysis based on strictly playoff results is going to be rife with small sample size problems...you can come up with pretty much any conclusion you want based on a sample of a few series.
I don't know if you read the book (Moneyball) but the main point that I took away from it was that the Oakland A's roster was made to win games.
The whole point of the book was that winning games was done by getting on base and the people who get on base more were better to have on a team than someone who was good at fielding (which the book said didn't matter).
The reason the playoffs were such a crapshoot wasn't because Oakland had a worse defense or pitchers but because the amount of games being played was so small that the results could go either way. as the teams in the playoffs were generally of similar quality.
In short, the A's roster was actually constructed to succeed over 162 games but the compromises he had to make on defense and power pitching to get extra value are particularly fraught in the playoffs, where you're battling the better offensive teams in the league. Of course, the playoffs are a crapshoot (the Cardinals had no business being in, let alone winning in 2011), but the compromises Beane had a direct effect on their heartbreaking playoff losses in the early 2000s.