Because real had a better team which could defeat the best opponents in Europe. Barcelona had a better system, which is essential when going up against weaker sides. I would say that’s the biggest difference. Real could figure out all the hardest puzzles but had difficulties against weaker sides. Barcelona, the opposite. That’s also why city won last year, as stated by pep himself. They diverged from their system and became more flexible. They were also less dominant in the league but that’s part of it.

Also to be noted that Barcelona has bribed referees, and a couple seasons have been very close and such things can have big consequences