Both times they made the World Series, they played poorly in the series overall. It wasn't necessarily because of defense and bullpen. I don't know that having sub-par defense or bullpen makes a team more prone to losing in the World Series than having a weakness in another area.
As others have pointed out, the bullpen played a big role in losing to the Red Sox in 2013, but they would not have gotten as far as they did that year without strong hitting and starting pitching. I long ago (maybe since 1987!) gave up trying to figure out why teams win or lose in post-season. It just seems to be a matter of getting hot at the right time.