Why Anand and Carlsen insisted on playing the same opening/defense They played the Berlin defense and Ruy Lopez. Well the question is more like: why Anand insisted to play Ruy Lopez again and again. wouldn't be better if he had tried najdorf and nimzo indian earlier? I felt like Anand is giving Carlen the world championship as a thanksgiving gift! Why would a GM in a world championship be so stubborn. What was he hoping for and what was his plan by insisting to play the same opening?
One of Anand's strengths relative to Carlsen is better opening knowledge, and Anand also had the better analysis team. (UPDATE: According to Carlsen in a recent interview, his team of seconds during the Chennai match consisted of Hammer, Fressinet and Eljanov.) To repeat openings was a natural match strategy for him, trying to turn the match into an opening discussion.

By varying openings, especially with white, Carlsen cleverly avoided most of Anand's preparation. The only set-up that was repeated was d3 against the Berlin (played in two white games by Anand), but Anand couldn't find any weakness there. Anand has played 1.e4 most of his career, and it's understandable that he tried to catch Carlsen there. The one game Anand played 1.d4 he lost, although probably from a strong position.

Yes, Anand should have taken more chances, but even with hindsight it is difficult to devise a  match strategy for him leading to success.