Barca's strength is and always has been from the Midfield triangle. All these people rave on about Messi being the greatest. He is not, in fact his success can be more attributed to the like of Xavi and Iniesta. Now do not misunderstand me, Messi is an amazing players, Ive seen him get the ball 25 yards out dribble his way through 3-4 defenders leaving them all dizzy then scoring a goal, definitely a player that will leave his stamp on a generation. Now back the original point, Without the like of Xavi, Iniesta and a lesser degree Busquets, Messi wouldnt score half the goals he does, Xavi is a passing genius and Iniesta is one of the most unique midfielders ive seen, In tandem then wreck more havoc imo then Messi does on his own, the evidence being is that poor messi gets stuck shouldering all responsibility for Argentina. Xavi and Iniesta win titles at club and national level because they are foundation of success a team can be built (Like 2 CBs can be, for example Vidic and Ferdinand circa 2008), Messi cannot (Or hasnt yet) do it by himself.
My ramblings aside, Its definitly because there midfield is the best in the world, one of the best in history. Madrid can spend millions on Ozil, Ronaldo and Kaka but they will never be able to give the experience that Barca's youth system did for Xavi and Iniesta, How do you challenge a partnership and relationship thats been there for years, you cant.