The reason is because Barcelona is a better 'team' than Real. Simple. Real is more top heavy stacked talentwise, so if you gotta play one game against them chances are you'll lose. This was really evident when Juventus actually wanted Real in the semis as opposed to the final. They believed they could beat them over 2 legs but not in a winner take all. Barcelona is built for the long run, not as a 1 stop monster. They'll always win more long term titles but they aren't quite as elitely built to win championships in small windows like real is. A great example if you watch basketball would be the Spurs. Barcelona is built like the Spurs, and if you watch football is Barcelona is built like the Steelers. Great long term, win the division but just not as top heavy as Real who when they're on form will destroy